Alexander M. Haig Jr., President-elect Ronald Reagan's choice to become secretary of state, said yesterday that previous investigations of his years at the White House as an aide to President Nixon had turned up nothing that would disqualify him for his newly proposed position.

But in order to prepare for an expected new look into his past during confirmation hearings next month before the Senate Foreign Relations Committee, Haig said he had taken up the offer of an old friend -- prominent Washiington lawyer and Democrat Joseph A. Califano Jr. -- to help him.

Appearing at a crowded Washington news conference, Haig was asked if he had any regrets about the advice he gave or actions he took regarding wiretaps, Watergate or the bombing of Cambodia during the Nixon years.

"Let me set the record straight," the retired four-star general responded.

Referring to "a number of recent reports by our own press here in Washington and elsewhere" dealing with his involvement in recent American history, Haig said, "I think it is important for all of us to keep in mind that I have appeared before several grand juries, a Senate investigating committee and a court of law on these subjects and nothing during that period indicated any culpability on my part."

"That has been reaffirmed recently," Haig continued, "by Mr. Leon Jaworski, the chief prosecutor at the time" of the 1974 Watergate investigation. Jaworski, once critical of Haig in his role as White House chief of staff, said this month that the nation actually "owes Gen. Haig a debt of gratitude" for steering Nixon toward resignation instead of a divisive impeachment trial. Jaworski called Haig "one of the unsung heroes of Watergate."

Haig said he was prepared to answer questions again "across the full gamut" of concerns but would give his answers at his confirmation hearing, not a news conference.

Although Haig was officially nominated last week, he appeared here yesterday as part of a news conference called to announce appointment of four new Cabinet members, and his presence tended to dominate the podium and the questioning.

Haig met for an hour privately in the morning with outgoing Secretary of State Edmund S. Muskie yesterday and resisted reporters' questions about what he would do on the Iranian hostage stalemate.

Haig said he was just starting to get filled in on a number of highly sensitive and important foreign policy issues and that "at this juncture we still have one president and one secretary of state and I think it behooves all of us to rally behind them." Haig said he didn't intend to offer any observations of his own now, but when asked if he shouldn't send some kind of signal to Iran about the Reagan administration position, Haig said he preferred to know more but that "I am generally inclined always to support the president and secretary of state on important issues of this kind."

To a question that reflected concerns about his military background, Haig said military men know better than others "the dreadful sacrifice of conflict." He said that he hoped he could follow in the footsteps of another general and secretary of state, George C. Marshall, and "make a somewhat comparable contribution" to global stability.

Asked if he saw the State Department job as a steppingstone to the White House, Haig said he didn't know anybody who suggested that State was a road to the presidency since the end of World War II. Many secretaries, he said, left in controversy. He said he was not leaving his comfortable corporate presidency at United Technologies Inc. "with any other objective in mind but to recognize that this country faces a number of very urgent and dangerous situations" which he hoped to handle successfully on behalf of Reagan.

"It goes without saying," Haig declared, "that America today has experienced a number of setbacks internationally, not only among longstanding allies but among countries of the Third World upon whose goodwill we will vitally depend. There is a great deal of work to be done. America must, in the period ahead, indeed we will one way or the other, make a decision as to whether or not we are going to continue to seek a world structured on the Christian-Judaic values that you and I cherish today, or whether we are going to turn that task over to the others."

Asked how he was preparing for his new job in the weeks ahead, Haig dealt rather good-naturedly with the added burdens that concern about his past was now causing him.

First, he said, was the work involved in "decoupling" himself from the $12-billion-a-year company he headed. Then he said, he was faced with the "immense task" of filling hundreds of vitally important "billets" -- a military term meaning jobs that undoubtedly shook up any State Department employes listening -- plus picking his own staff.

Beyond that, Haig said, was the job of getting "read in" on many "highly sensitive and highly dynamic" foreign policy issues ranging from the critical situation in Poland to the Middle East, Central America and relations with the Soviet Union.

All of these are time-consuming, he said, but there was also the matter of answering "the host of questions that have arisen very recently from some sources of past history associated with the Watergate affair."

Haig said he met over the weekend with Califano on this aspect of his preparation. There was no indication of how extensive Califano's efforts would be. Califano, who served in the Kennedy, Johnson and Carter administrations and is now a partner in the law firm of Califano, Ross and Heineman, issued only a one-line statement that he was assisting Haig and was unavailable for telephone inquiries.

Califano was one of Haig's earliest and most important boosters, focusing on him for a key Pentagon post in 1963 as one of a new wave of "Renaissance men" in uniform.