Defense Secretary Caspar W. Weinberger began his testimony to the Iran-contra committees yesterday and made these points. ON MODERATE ELEMENTS IN IRAN

Weinberger testified that he dismissed from the outset the Reagan administration's secret initiative to improve relations with Iran, calling it a bad idea based on a faulty premise that a "moderate" element existed within the Islamic revolutionary government.

"I didn't think there was anyone we could deal with that was not virulently anti-American," Weinberger testified about the "moderates." He also faulted the National Security Council staff for stringing the president along with favorable reports about the outlook for hostages to be released and singled out former national security adviser John M. Poindexter for criticism. ON LEARNING OF THE ARMS SALES TO IRAN

Weinberger testified that the first inkling he had that U.S. and Iranian officials were negotiating potential sales of arms in 1985 came not from the White House but from an intelligence agency under Weinberger's control. He said he had similar experiences in 1986 in first hearing of White House moves in the Iran initiative. ON THE POTENTIAL FOR BLACKMAIL

In meetings before the Iran initiative began and after it was disclosed, Weinberger testified that he argued that selling arms to Iran exposed the United States to the possibility of "blackmail."

"We must bear in mind we have given the Israelis and the Iranians the opportunity to blackmail us by reporting selectively bits and pieces of the total story," Weinberger said he commented last Nov. 10.

Weinberger testified that his objections to the Iran initiative were ineffective. "I was not able to be persuasive enough, and I'm sorry that I wasn't."