French Foreign Minister Claude Cheysson said today that this country's new Socialist leadership will prove a "reliable partner" for the Atlantic Alliance and expressed hope that the United States will avoid past mistakes of distrusting French leftist goernments and thereby hampering their effectiveness in their crucial early months.

In his first interview as foreign minister, at a breakfast meeting with The Washington Post, Cheysson outlined in broad terms a foreign policy that appeared to confirm assessments here of a broad area of accord between Paris and Washington on Middle East and East-West issues and potential disagreements on the Third World, especially Central America, southern Africa and North Africa.

In apparent reference to the way France's previous government under former president Valery Giscard d'Estaing tended to catch American officials by surprise on undiscussed disagreements, Cheysson said the United States will find that it likes the open, direct way that the Socialist government of President Francois Mitterrand intends to talk things over.

Prominent Socialists here have been worriedly asking Americans in Paris what they can do to reassure a Reagan administration that is widely assumed here to be at least skeptical and potentially hostile, if only because of the presumed ideological incompatibility between socialism and conservative Republicanism.

"The Americans have to give us three months," Jacques Delors, the Socialist moderate who is France's new finance minister, said yesterday. "Mitterand is not Michael Foot," the leader of the increasingly left-wing Labor Party in Britain. "The Americans must understand that."

Cheysson today recalled that the effectiveness of the government of Pierre Mendes-France, in which he served as executive assistant, was damaged in 1954 by the distant treatment it got during its first three months from the Eisenhower administration until U.S. Secretary of State John Foster Dulles learned to trust Mendes-France.

The new foreign minister said Washington's suspicion at that time had led to what he called "maneuvers against the French franc" -- a recollection that came at the end of a week in which the Bank of France is said to have spent at least $4 billion in a not very successful effort to defend the value of the franc against the U.S. dollar.

So far, French Socialists have been carefully ambiguous about the question they know is uppermost in the mind of the Reagan administration -- will there be Communists in the Mitterrand Cabinet after parliamentary elections officially set today for June 14 and 21. It appears to be a subject of internal debate among the Socialists, with the outcome likely to depend on how well they do in the elections.

The new rulers of France cannot have been very reassured by statements of embassy officials here from the Soviet Union and the United States. To the last moment, officials from both embassies were saying they expected Giscard to be reelected. In addition, top U.S. Embassy staffers showed an overtly pessimistic, wait-and-see attitude following Mitterrand's victory.

"You don't want to believe it when it's something you don't want to happen," one State Department veteran here said of the election.

But he noted that the U.S. administration bounced back with Reagan's day-late congratulatory message in which the White House included a particularly warm personal comparison between Reagan's own persistence in winning the presidency and Mitterrand's.

Reagan is believed to be sending still another personal message to be hand-delivered Sunday by West German Chancellor Helmut Schmidt, who is stopping here on his way home from Washington to mend fences with a fellow Socialist leader he had generally neglected in furthering his close relationship with Giscard.

"In terms of the Atlantic Alliance," Cheysson said in his impeccable diplomat's English, "you can't have a more reliable partner than us."

But disagreements, he said, especially over Third World problems, are inevitable.

Cheysson said he cannot understand why the United States, "the greatest country in the world," made a test case out of El Salvador in which Western Europe was expected to "kneel" to U.S. policy and opinion. He noted that opponents of the Salvadoran government of Christian Democrat Jose Napoleon Duarte have ties with European Christian Democratic and Socialist parties.

He said that the Reagan administration views the Third World as a problem of "charity" and "respect for the starving," but regards it as marginal in most strategic terms.

"For us," Cheysson said, "the Third World is an essential partner; we depend too much on them for our supplies."

In his previous job as the French commissioner in the executive of the European Common Market, Cheysson was in charge of relations with the Third World. He was largely responsible for expanding to 60 Third World countries the Lome Convention that had tied Europe to a narrow group of former African colonies through guaranteed raw materials prices and markets.

One way Cheysson indicated that he intends to be reassuring is to make the minimum number of changes of French ambassadors.

"We are completely new and strange to our allies," he said, so it is important not to upset them by changing the diplomatic representatives they were used to dealing with. He specifically included the French ambassador in Washington, Francois de Laboulaye, among those who would not be moved at least for the time being.

The minister also noted that in the Cabinet list, announced yesterday, his ministry's name was formally changed from foreign affairs to external relations with various parts of the world, especially France's former African colonies.

The old African Cooperation Ministry has been placed for the first time under the professional diplomats of the Quai d'Orsay with young Socialist Jean-Pierre Cot as Chyesson's ministerial-rank deputy for France's African aid program. From now on, Cheysson said, French relations with its former colonies will be treated as part of the whole range of relations with the world.

Although he refused to get into specific difficulties with particular countries, it is clear that the Socialist government plans to strengthen ties with socialist Algeria at the expense of Morocco, which as been close to Washington and Paris under King Hassan II. The new French government is also planning to keep at arm's length governments like that of Zaire's Mobutu Sese Seko, with their reputations for right-wing authoritarianism among West European Socialists.

Asked what that would do to Franco-American relations, a Socialist Party African affairs specialist replied that Washington would simply have to assume more responsibility for governments like Morocco and Zaire, where the Americans had come to rely on the French lead. But this specialist said, contrary to some expectations, that Mitterrand can nevertheless be expected to strengthen rather than abandon the French intervention forces that Giscard used primarily to defend French-speaking African allies.