William Steele Sessions, President Reagan's nominee to head the Federal Bureau of Investigation, was asked at a news conference yesterday about his reputation as a "West Texas tough guy."

Sessions, a silver-haired 57-year-old with a folksy manner and a reputation as a tough sentencer as a federal trial court judge in San Antonio, smiled. "I love the accusation," he said.

"I don't wear a gunbelt. And I don't have any cowboy boots to my name," said Sessions, dressed for his first day as FBI director-designate in sober gray pinstripes and a maroon-stiped tie. "If I'm a West Texas tough guy, it's simply because we have dealt with some very difficult problems out there."

Sessions worked here from 1969 to 1971 as chief of the now-defunct government operations section of the Justice Department's criminal division, prosecuting obscenity, draft-evasion and election-fraud cases. He then served as U.S. attorney in San Antonio before being named to the federal bench by President Gerald R. Ford in 1974.

Attorney General Edwin Meese III said Sessions has "seen the FBI in action . . . . I don't think it's any mystery to him as to what he's getting into."

Sessions, however, professed a "wealth of ignorance about the FBI" and said he has "grave misgivings" about leaving the bench. Still, the Fort Smith, Ark., native said, "the attraction of being associated with people with the white hats is strong."

Sessions and his wife, Alice, have three sons -- two are trial lawyers in San Antonio and one is a Bell Corp. representative in New Jersey -- and a daughter, a ballet dancer just out of high school. The Sessions, who met in high school in Kansas City, Mo., have four grandchildren and a fifth on the way.

"The consideration of leaving them behind to come to Washington was very great to me," he said.

Meese said the administration, in moving to replace former FBI director William H. Webster, a former judge who is now CIA director, "wanted a clone of Judge Webster." With Sessions, he said, "We came as close as we could. He answers to 'Judge.' He answers to 'Bill.' . . . The retraining should be simple."

Sessions seemed nervous as Meese introduced him to the news media and described the four-month search to fill "perhaps one of the most important positions to which the president will appoint anyone."

But he was relaxed and joking in answering questions, starting out by saying his day so far, meeting with Meese and Reagan, "was vigorous enough that I came down here to rest."

Sessions declined to answer specific questions about his plans at the nation's chief law enforcement agency, other than promising integrity and fairness. But he said he is "keenly aware" of various constitutional rights, described drugs as a "true menace" and assailed pornography as "vicious stuff."

"I do not look on it as a victimless crime," he said.

Sessions, who is leaving a position with lifetime tenure, said he is not concerned about signing up for the 10-year stint as director. "I'm 57 years old," he said. "The thought of being locked in doesn't worry me. I hope I can serve with enough distinction that it will be regrettable that I have to depart."

Sessions is an avid canoeist and hiker who has made it 18,000 feet up Mount Everest. The Dallas Morning News reported in January 1986 that he had applied to be one of the civilians chosen to ride the space shuttle, but NASA rejected his bid to be the first judge in space.

Being named to the FBI post may ground Sessions, however. A reporter asked whether Sessions had promised Reagan that he would give up mountain-climbing. "I am now a trekker and that's it," Sessions declared.