GARRETT IS the westernmost of Maryland's counties. To get there via Interstate, take the Beltway to I-270 north, to I-70 west, to U.S. 40 and I-48 west to Grantsville or to U.S. 219 south to Oakland and Deep Creek Lake.

Or, take U.S. 50 west through Virginia and West Virginia to U.S. 219 and turn north to Oakland and Deep Creek Lake. The distance is about 150 miles, drive time about three hours.

A Vacation Guide, free from the Deep Creek Lake-Garrett County Promotion Council, is a prime source of information for almost everything in the county. It is free at information centers in Keysers Ridge, on U.S. 219 at I-48; and at Deep Creek Lake on U.S. 219. Or, from the promotion council (Oakland, MD 21550; 301/334-3888). The promotion council office is helpful in locating housing. It also publishes a 50-cent tour map and has copies of Maryland Geological Survey topographic maps for $2.50.

Tourist services are clustered at Deep Creek Lake, though there are some in Oakland and in Grantsville at the Casselman Inn. Around Deep Creek Lake there are motels, chalets, condominiums and rental cottages.

Prices range from $25 a night at the Oakland Motel to more than $100 per night at the Will o' the Wisp at Deep Creek Lake. The Vacation Guide provides an excellent list of accommodations with prices.

There are campgrounds in state forests and parks. Park campgrounds, with the notable exception of Big Run State Park, usually have sites with electricity and water. At Big Run and in the state forests, the sites are primitive. There are also several private campgrounds -- listed in the Vacation Guide.

On the whole, eating in Garrett County is good and plentiful and cheap by city standards. Most restaurants serve fresh baked bread, for instance, and they all serve huge quantities.

The Penn-Alps in Grantsville is outstanding on all counts. The Topsider in McHenry near Deep Creek Lake has great hamburgers and steaks, though the prices are closer to city standards. The Oak-Mar restaurant in Oakland has great vegetable soup and a tasty and generous turkey and dressing special every Saturday and Sunday.

Other restaurants that local folk recommend include the Silver Tree Inn and the Will o' the Wisp at Deep Creek Lake and the Cornish Manor in Mountain Lake Park. There is not a single McDonald's in Garrett County.

For easiest travel, return early on Sunday or stay over until Monday. Every summer Sunday evening, traffic is very heavy on I-48 and I-270.

Three books will make your visit more interesting: ''Where the Potomac Begins" by Gilbert Gude, "Youghiogheny, Appalachian River" by Tim Palmer, and "Garrett County" by Stephen Schlosnagle, though the last bogs down from time to time.