"I didn 't believe it, I just didn't believe it," Arizona Court of Appeals Judge Sandra D. O'Connor said today after President Reagan nominated her as the first woman justice of the U.S. Supreme Court. The president had called her Monday afternoon, informing her of his decision, she said during a morning press conference. "This is a momentous day in my life and that of my family. I am extremely happy and honored to have been nominated," she told reporters. "IF CONFIRMED, I will do my best t.o serve the court and this nation in a manner that will bring credit to the president, to my family and to all the people of this great nation." Mrs. O'Connor, 51, considered a moderate-conservative, declined to answer issue-oriented questions from the press until her U.S. Senate confirmation hearings. According to Reagan aide Peter Roussel, no date has been set for those hearings. In announcing his choice for the high court vacancy created by the retirement of Justice Potter Stewart, Reagan noted the speculation that was raised concerning the appointment of a woman. "I MADE A commitment that one of my first appointments would be the most qualified woman I could possibly find," Reagan told reporters at a hastily-called news conference at 8 a.m. Phoenix time to answer specific issue questions, but that didn't deter reporters from asking politically sensitive questions. MRS. O'CONNOR, described by associates as a moderate-conservative, refused to categorize herself on the political spectrum.