1. Know your points!
In football, a touchdown is worth 6 points, a field goal is worth 3 points, and a two-point conversation which is the team's second chance to get points after they score a touchdown is worth 2 points. There is also a safety, which means the player with the ball is tackled in their own end zone and the opposite teams gets 2 points.
2. Know the quarterback!

3. Know your downs!
You will hear the announcer say "2nd and 10" or "3rd and 5" which is the number of down the team is on and how many yards they have until they get a 1st down. Each team gets 4 downs to get a touchdown and if they don't they turn the ball over to the other team.
4. Know about the magical yellow line!
The most important thing to know about the yellow line you see on TV is that the players can't see it. It basically just shows you how far the team has got until a 1st down. It is very important to not ask how the yellow line moves around on the field so much.
5. Know who's on offense and defense!
It sounds dumb but you don't want to be the girl that is screaming for the wrong team. Yelling, "Get the ball!" when your team is trying to score can totally blow your cover. If your team has the ball then they are on offense, if not they are on defense.
While there is still so much more to the game this is enough to have you covered for at least a while. When in doubt, ask some questions so at least it seems like you are interested. There is also the option of ditching the sports lingo and just keep grabbing the guys beers and you won't have any issues being the "cool" girlfriend.
Hope this helps next Monday night!