Joe Theismann, who will turn 30 a week from today, admits that he acted like a fool at times in the past and also concedes that many of his mistakes during football games were a result of not understanding the offensive system.

"I feel very much at home and I'm comfortable now," Theismann said on the eve of the Redskin season season opener today against the Houston Oilers.

Despite his understanding of the philosophies of Joe Walton, the offensive coordinator, Theismann made enough errors in the final exhibition game against Cleveland that the fans at RFK Stadium booed him loudly.

"That's just their way of showing they love ya," Theismann said.

"You try not to pay attention to it. You can't deny hearing it. Nobody likes to get booed. I heard a couple of cheers, too, once. So maybe it'll offset one another. I sure would like to have more cheers than boos, though."

Whether Theismann gets cheers or boos from the fans, he must above all impress his teammates. With the Redskin youth and inexperience, he must establish himself as a team leader by his performance, particularly early in the season.

"A lot of our success will depend on how he plays initially," said Bobby Mitchell, the club's executive assistant to Edward Bennett Williams, the team president.

"We're talking about the first three or four games,"Mitchell continued. "Young kids haven't been around enough like the old vets to think that they can get it together later. If Joe can get off to a good start and fire up these young kids, we'll be okay."

By okay, Mitchell said, he means better than a .500 season.

"I don't want to think about anything else but Joe getting off to a good start," Mitchell added, "because anything else scares me to death. I think he's ready mentally. He feels it's his team and he has to turn it on."

Theismann knows he must turn it on by his deeds.

"You don't have to put on a show or anything like that," he said. "You've got to go out and do your job and if you do it the best you can, then the guys will respect you. I would never claim to be a great philosopher. Sometimes I talk too damn much, to be honest with you.

"I know that, but I think I've grown up in that sense too. There's a time to say things and a time not to say things. I think hopefully a wise man knows when not to say things; sometimes a fool doesn't know when to shut up. I've probably been both of them."

In the past, Theismann also says he was thinking too much under pressure instead of merely reacting to the situation and what the system called for.

Now, after a season of starting, an off season spent with Walton and a second training camp under the system, Theismann says he now understands the offense "50 percent better.

"Through training camp, I just tried to familiarize myself as much as I can with our offense," he said. "I want to understand his (Walton's) offense a lot better and this year in training camp, I feel like I accomplished that. There's so much more to learn, but I think I've got myself pointed in the right direction."

Physically Theismann has tried to gain strength and weight.

"I wanted to get a little bit stronger and a little bit heavier and hang in," he said. "I'm heavier; I don't know about stronger. I have according to the weight charts, percentagewise, moved myself up a bit. I feel real good. I feel very comfortable and I feel very optimistic about our football team. I think everybody has had an opportunity to work a year in our system."

The avalanche of newcomers last season, after Allen was fired, was one reason Theismann said he turned to tight end Jean Fugett so much in tight situations.

"He has a feel for it," Theismann said. "Jean and I have been working together for three years . . . That's why training camp is of such vast importance. It's not like a light switch. You can't go out and say, 'Everything's going to click. Okay, now turn it off for awhile.' It doesn't work that way."

And Theismann is the first to admit there is plenty of room for his improvement:

"It's just a matter of execution and continue to learn and continue to concentrate. The biggest thing is to continue to work. I've tried to break down my mistakes and not make the same ones again. I feel as far as our system goes that I'm a good 50 percent ahead of myself as far as understanding and knowing what to do with the football."

The basic better understanding of the offense will in itself help eliminate such mistakes as picking out the wrong receiver or taking a sack when he should have thrown the ball away, or running and scrambling too soon or too much, the quarterback says.

"That comes from learning the offense," Theismann said. "I mentioned before knowing what to do with the football. You have to be aware of the down and distance. It's sort of foolish to take a sack on first and 10 when you have two more downs to pick it up. That kind of stuff. That's what I talk about when I say reacting instead of thinking.

"Those are the elements and those are the parts of my game I want to work on and continue to work on. You see everybody eat the ball every now and then. I've run with the football a little bit.I've been very fortunate as far as my running has gone. It's been a plus for me.

"Sometimes I instinctively take off when I shouldn't. What I'm trying to do is make the right decision. When to go and when not to."