About 15 minutes into Jody Powell's debut yesterday as the press secretary to a President, a veteran White House reporter mumbled, to no one in particular, "This is the first funny briefing since the Johnson administration."

Powell was in the midst of reciting some of the minutely detailed aspects of a President's day that the White House press corps often demands to know. He announced that the last members of President Carter's family to return to the White House after the maugural festivities were the President's son, Jeff, and his wife, Annette. The couple got in at 5 a.m., he said, after leaving friends from Georgia "at an undisclosed place in Virginia."

"I'm not sure Jeff didn't want to disclose it or was unable to remember it," Powell added.

The 33-year-old Powell, one of Carter's closest aides, is as slick a downhome country boy as can be found, even in Jimmy Carter's Washington. His quick wit can be combative when challenged, or can turn into a gentle, self-mocking humor from which not even his boss, the President, is immune.

For his debut yesterday Powell chose the latter course, perhaps hoping to set a tone that will not last forever, or maybe even long, but could ease the first few days of a new administration. He was, no doubt, also helped along by the inevitable "honeymoon" aspects of the press corps' relationship with a new President, with even the most notoriously outspoken White House regulars quietly scribbling down his every word.

Powell arrived for the briefing at 11:45 a.m., 15 minutes late, which by normal White House standards and his own record during the Carter campaign was practically ahead of schedule.

He faced what was probably the largest audience to cram into the White House, briefing room since the dramatic last days of the Nixon administration in 1974. Reporters sat jammed together on sofas, tabletops and the floor.

"Can we get a larger briefing room?" someone asked.

"I'm hoping it will be so uncomfortable that no one will come," Powell replied.

In the past, Powell has made little attempt to hide his disdain for questions about what he considers "trivia." But yesterday, for the first full day of the Carter presidency, he came around with just such details.

"Okay, trivia from this morning," he said. "The President, in his words, 'slep late.' He got up at 7, read two or three newspapers. He couldn't find his news summary. We'll try to get it for him in the morning."

THe President's breakfast, he allowed, consisted of "scrambled eggs, toast, sweet rolls - no grits - sausage and bacon."

Before breakfast, Powell said, the Carter family discovered that there was no high chair for Jason Carter, Jack and Judy Carter's 17-month-old son and the President's only grandchild. They improvised by getting a chair from the White House billiard room, which Powell said "will be converted to the pool room" in the Carter administration.

"Billy was there," he added, speaking of the President's brother who revels in his image as a beer-swilling rednect. "He said he had heard what a high-class place this was but the damn pool table doesn't have any pockets," Powell said.

So it went, for the first 25 minutes of Powell's first briefing as White House press secretary, in which he proved that humor can be a disarming weapon. He was asked about the qualification of Hugh Carter Jr. - the President's cousin once removed - to be special assistant to the President for administration.

"If you're the President's cousin, do you need any?" he said. The subject was dropped.

"Another bit of trivia . . ." Powell said at one point.

"This is not trivia," a reporter said.

"Okay," Powell replied, "I was embarrassed by it and I trust you all would be too . . . Another historical bit of information," he added, after which he announced Jeff and Annette Carter's early morning adventures.

In the midst of all this, there was some serious news. In addition to Hugh Carter Jr.'s appointment, Powell said that Richard Harden, an aide to Carter when he was governor' of Georgia, would be special assistant to the President for budget and organization, and that Greg Schneiders would be director of White House projects.

Schneiders, a close Carter campaign aide, was in line to be Carter's appointments secretary until a FBI check found personal financial problems in his past.

With great relish and an accompanying fact sheet ("height-32 1/2", length-72", width-48"), Powell also announced that Carter selected the "Hayes" or "Resolute" desk for his personal use in the Oval Office. This desk, now at the Smithsonian Institution, was last used by President Kennedy.

It was not until 12:10 p.m. yesterday that Powell's first White House briefing turned serious. It was then that he announced the President's proclamation for Vietnam war draft evaders. Many questions followed in the next hour, and for some he had no answers. But Powell, with his engaging manner and quick wit, had still managed to get through his first full White House briefing with few, if any, scars.