President Reagan gave Mexican President Jose Lopez Portillo a warm welcome at the White House yesterday and then escorted his guest to Camp David for talks that centered on new plans for the Caribbean basin and for closer bilateral U.S.-Mexican relations.

In the relaxed atmosphere of the presidential retreat in Maryland's Catoctin Mountains about 60 miles from Washington, the two presidents and their top advisers plus almost their entire cabinets were also expected to discuss the fighting in El Salvador, on which the two nations differ, immigration of Mexicans to the United States and trade.

Reagan began by giving new emphasis to the important role good relations with Mexico and Canada has in his foreign policy.

"In a world filled with neighbors who resort to violence, neighbors who've lost sight of the shared values and mutual interests, the good will between Mexico and the United States is a blossom whose beauty we meet here to cherish and protect."

Reagan and Lopez Portillo each addressed the longstanding sensitivity of Mexico over being treated as a junior partner by the United States.

"I welcome you today with the pledge that this administration will sincerely and dilegently strive to maintain a relationship of mutual respect and cooperation between our tow nations, and the decisions which affect both sides of our border will be made only after the closest consultation between our governments," Reagan said.

Lopez Portillo said there should be neither arrogance nor submission in the relationship between the two countries. "We want to be understood and we want to understand," he said.

In 1979 Lopez Portillo lectured President Carter on the arrogance of U.S. attitudes and refused to embrace the American president when he visited Mexico City. Yesterday Lopez Portillo and Reagan greeted each other with the traditional Latin American embrace.

Lopez Portillo is the first foreign leader Reagan has invited to the presidential retreat at Camp David. After the short helicopter flight there, the two men changed into casual clothes and met, with interpreters, for 70 minutes.

Then they joined their top foreign policy aides for a two-hour meeting that included lunch on the patio over-looking a meadow and a swimming pool at Aspen Lodge. A senior U.S. official told reporters via a telephone hookup to the White House press room that most of the two meetings was devoted to Reagan's evolving terlocutory role Mexico can play in the region.

The official, who spoke on condition he not be identified, made the meetings sound as though they had been an extremely general exchange. For example, he said Lopez Portillo gave "a sketch" of Mexican development that included such details as the nation's population (70 million), its area (764,000 square miles) and its rate of population growth (down from 3.6 percent annually to 2.8 percent).

In the same vein, reporters were told that the two presidents agreed that economic development "should be a goal for the whole region."

The senior official gave no indication that there had been much disagreement between the two presidents.

Reagan supports the ruling junta in El Salvador; Mexico disapproves of the U.S. military aid Reagan is supplying it. Mexico supports the Sandinista government in Nicaragua; the United States in withholding aid from Nicaragua to pressure the regime.

Asked whether Lopez Portillo had voiced his much-publicized position that both the United States and the Soviet Union should stay out of El Salvador, the senior official said not in "precisely those terms."

Although the administration doesn't yet have a finished plan for the Caribbean, it is working toward a program that would seek to provide long-term economic support to increase development in Central America and the Caribbean island nations.

Lopez Portillo sees a prominent role for Mexico in such a plan, the official said, as a nation that has good relations throughout the region -- with Cuba as well as the United States.

The two presidents did not discuss any Mexican go-between role to facilitate U.S.-Cola Communications, the official said.

The two presidents ended their first day by going horseback riding and having a barbecue