1) Yell "fumble" when there is a fumble.
This shows that you are really into the game.
2) Never call an Extra Point a "field goal".
Extra Points or Point Added Total (PAT), when kicked, are worth 1 point versus 3 points for a field goal.
3) If the quarterback (QB) gets hit hard and/or sacked, comment on it.
"Dang! He's gonna feel that in the morning!", or if your QB gets sacked, "Darn! No protection on that play!" or "He's gotta get rid of the ball quicker."
4) If the opposing QB has the ball and the defense can't get to him, say:
Something like "Too much time!" or "Get him!" or "Will someone get the QB?" Shows your frustration with the lack of effort or talent the defense may have.
5) Your team's running back runs up the middle for a big gain.
"Great run! He punched through that hole." or "Our linemen are earning their keep." It tells other people that you understand the offensive linemen are vital in those kinds of plays.
6) Your team gets an "interception"
"Nice pick!" or if they run it back for a touchdown, "Man! I never get tired of seeing pick-6s!" Of course a pick meaning interception and pick-6, an interception returned for a touchdown.
7) Cheer when your offense scores
Be excited for a touchdown. Yelling "touchdown" or a celebratory sound works. Be "ok" with a field goal. You don't need to say anything, since it's similar to a consolation prize (unless it's the game winning kick). If you feel compelled to say something, try, "Well. At least we got some points on that drive."
Remember, a lot of sounding like you know football and sports in general, is commenting or yelling about the obvious. You don't have to do play by play like Joe Buck or Troy Aikman talking about Tom Brady's arm or Adrian Peterson's running ability.