Only the hardiest outdoor types pitch their tents in winter-buffeted campsites; most of the nation's 61.5 million campers are eagerly awaiting spring. But it's not too early to plan ahead: Competition for campsites has increased tremendously in the last two decades. In fact, the number of campers in this country has risen 500 percent since 1960.

But there is a wide selection of campsites offered by federal, state and local governments, and by commercial interests, for individual or family camping, for tents or recreational vehicles. Among the choices are:

* National forests. The 191 million acres supervised by the National Forest Service contain 7,000 developed campgrounds, 100,000 miles of trails and 83,000 miles of streams and rivers. Campers are not limited to the campgrounds -- tents can be pitched in most areas of most forests, provided that permits are secured when required. Write to specific forest supervisors for information on camping. A free list of all the forests with addresses is available from the Forest Service, U.S. Department of Agriculture, Office of Information, P.O. Box 2417, Washington, D.C. 20013.

* National parks. There are 105 areas with camping facilities within the 48 national parks. A free color guide (include 75 cents for postage and handling) to both the national forests and national parks is offered by the Recreational Vehicle Industry Association, a national organization representing manufacturers and suppliers of recreation vehicles. Write: RVIA, FT, P.O. Box 2999, Reston, Va. 22090. In addition, RVIA has published an illustrated brochure, "National Park Service Travel Tips," which is available free from the Park Service, U.S. Department of Interior, Office of Public Inquiries, 18th and C Streets NW, Washington, D.C. 20240.

Two booklets -- "Camping in the National Park System" ($1.50) and "Lesser-Known Areas of the National Park System" ($4.50) -- can be ordered from the Government Printing Office, Superintendent of Documents, Washington, D.C. 20402.

* State and local parks. The travel and tourism bureau in each state capital can provide information about camping in state parks. A free list of state travel directors can be obtained from the Travel Industry Association, 1899 L St. NW, Washington, D.C. 20036. State and local campgrounds, as well as commercial sites, are listed in four comprehensive national campground directories available in bookstores and libraries: "Rand McNally's 1985 Campground & Trailer Park Guide," "Trailer Life's 1985 RV Campground & Services Directory," "Wheelers RV Resort & Campground Guide" and "Woodall's Campground Directories" (eastern and western editions).

For a free catalog explaining how to order one of these directories or more than 40 other related publications by mail, send a stamped, self-addressed long envelope to RVIA at its Reston address.

* National refuges. The U.S. Fish and Wildlife Service manages the National Wildlife Refuge System, made up of more than 400 areas in 49 states. The refuges are known for the variety of birds and wildlife (including endangered species) living within their borders or stopping off while migrating. Primary recreational activities in the system are fishing, hunting, birdwatching, photography and environmental education. However, limited camping facilities -- mostly primitive -- are available on some refuges; most have none, although private sites may be nearby. Free information on refuge camping from U.S. Fish and Wildlife Service, Division of Refuge Management, Washington, D.C. 20240.

* Bureau of Land Management recreation sites. This federal agency manages almost 300 million acres of land in the western United States for multiple uses, including scenic recreation sites from Alaska to New Mexico. Camping information from BLM, Public Affairs Office, 18th and C Streets NW, Washington, D.C. 20240.

* U.S. Army Corps of Engineers projects. Nationwide these projects (such as flood control, hydroelectric power production and navigation) are also the location of 91,000 campsites, all water-oriented recreation areas near lakes and rivers. For a free list of district offices that will provide specific camping information, send a stamped, self-addressed envelope to U.S. Army Corps of Engineers, 20 Massachusetts Ave. NW, Washington, D.C. 20314, Attn: Public Affairs Office.

* Commercial campgrounds. Nearly 8,000 of these campgrounds are operating around the country, often near popular recreation areas and resort cities. Commercial campsites generally offer more amenities than their federal and state counterparts. For details on each of these campgrounds, refer to one of the four national directories listed above under state and local parks.

* Rental vehicles. Twenty-four million Americans now do their camping in travel trailers, motor homes, folding camping trailers and other types of recreation vehicles, according to RVIA. More than 150 RV rental firms in this country and Canada will rent a variety of family camping vehicles. See the listing in the Yellow Pages under "Recreational Vehicles -- Rental and Leasing," or send $3.50 for a 70-page guide, "Who's Who in RV Rentals," to Recreation Vehicle Rental Association, 3251 Lee Hwy., Suite 500, Fairfax, Va. 22030. TRAVEL ADVISORIES:

The U.S. Department of State periodically issues public advisories giving the latest information on travel conditions around the world, and the most recent have ranged from requirements for visiting Gibraltar to a warning about travel to New Caledonia. Some examples:

* Last month the Spanish government moved to permit persons of all nationalities to enter Gibraltar from Spanish territory, or to enter Spain from that historic peninsula extending into the Mediterranean from the southern tip of Spain.

The border will remain open 24 hours a day, seven days a week, for passage of visitors and vehicles without merchandise. All Americans must have a valid passport. Drivers must show a valid driver's license, proper registration of the vehicle, and evidence of international auto insurance. Motor homes and animals are not admitted to Gibraltar. Parking facilities in Gibraltar and the nearby town of La Li'nea are scarce; cars left in unprotected spaces "are subject to vandalism."

* Sri Lanka's government has declared the entire Northern Province, and the Eastern Province from Batticaloa south to Arugam Bay, off limits to foreigners because of continuing racial violence and tension between the minority Tamils and the majority Sinhalese.

The State Department adds that the situation "remains volatile and potentially dangerous" in the Trincomalee area as well, and recommends that Americans also avoid traveling there at this time. Travel to Colombo and major tourist areas, including the cultural triangle (Anuradhapura, Polonnaruwa and Sigiriya), the hill country and beach resorts in the southwest "should be safe." Roadblocks and unexpected curfews may occur in parts of the southern two-thirds of the island.

Americans are advised to register at the American Embassy in Colombo upon arrival. Visitors should be extremely cautious about photographing anything that might be considered sensitive, such as bridges, airports, military installations or personnel, and government buildings -- including homes of prominent officials.

* Continuing hostilities between Iran and Iraq "do not preclude travel to Baghdad and most areas of Iraq." Travel is not recommended to certain areas of northern Iraq and the areas bordering Iran.

U.S. citizens are "strongly urged" to register with the U.S. Embassy after arriving in Iraq; it is located across from the Foreign Ministry Club in the Masbah quarter of Baghdad. With few exceptions, all visitors to Iraq must be sponsored, and travelers from the United States should apply for visas at the Iraqi Embassy in Washington.

* Hotels in Denmark will be fully booked from May 8 to 12 due to the Scandinavian Furniture Fair. Confirmed reservations are suggested if you plan to be there during this period.

* The same advice is given for those who intend to visit Seoul from Sept. 29 though Oct. 18, when the International Monetary Fund (IMF) will hold its annual meeting. Early hotel reservations and written confirmation are also urged for travelers who will be in Rome during November. The American Society of Travel Agents (ASTA) will hold its annual meeting in that city from Nov. 10 to 16. Thousands of agents will attend, and there will be both pre- and post-convention activities.

* The nationwide state of siege declared last May in Colombia remains in effect. Americans traveling in that country "should act prudently . . . utmost care should be taken in major cities and equal caution should be exercised if travel to rural areas is planned." The recent extradition of Colombian citizens to stand trial in the United States on narcotics-related charges may increase the possibility of violence against Americans in Colombia.

It is particularly advisable to avoid travel to the Departments (states) of Guajira, Putumayo, Arauca, Caqueta', Huila, Meta and Cauca due to security problems. Proper documentation should be carried at all times, and Americans should register with the U.S. Embassy in Bogota or the U.S. Consulate General in Barranquilla upon arrival in the country.

* U.S. citizens should avoid travel to the French territory of New Caledonia in the South Pacific "unless absolutely necessary" due to continued unrest and occasional violent incidents.

These advisories (not all details are included above) are subject to change or cancellation. For additional information, or queries about other areas, contact the State Department's Citizens Emergency Center, 632-5225.