Football is ez sport. Here is a quick startup:
1) What type of football are you interested in? Do you want to watch pro or college football with your friends and family? Do you want to play fantasy football (recommended only for advanced users)? Do you want to play recreational football? Rec football is absurdly easy to learn. Each position has a fairly narrow job, so you can learn them one at a time as you play them and gradually see the picture come into focus.
2) So you've decided you want to watch football with your family/friends or play fantasy football. The first step to either is deciding which team you want to be a fan of. There are three main ways to do that. First, who do your family and friends root for? I would suggest picking that team (or, at least, not their rivals) for your own health benefits. Second, which team is geographically closest to you? If you live in, say, Kentucky, you have a few options. I would suggest weighting how close a team is with how unlikable they are. If you live in the Bay Area, for instance, just go with the SF 49ers for your own sake, regardless of how close you live to Oakland.
3) Learning the rules. The basics in football are dead simple. Really the most complicated bit is the penalties. The basics: the goal of football is to advance the ball down the field. If you advance the ball all the way down the field (into the "end zone," which is usually colored or decorated slightly differently from the rest of the field), your team scores 6+1 points1 . The catch is, like rugby, you can't pass the ball forward2. You can hold the ball and run forward, and you can pass the ball laterally or backward (called a lateral in football). There is one exception: once time during each play, if a player on your team is behind where the football started that play (the "line of scrimmage"), you can pass the ball forward. This responsibility is almost always handled by the quarterback, who is usually a leader on the team and has a lot of agency over what types of plays are used (will your team try to run the ball forward or pass it?).
When you watch football on TV, there are usually two lines superimposed on the field by the TV station: a blue one and a yellow one. The blue one is the line of scrimmage (remember: where the ball starts at the start of the play). The yellow one is the line to gain, and is placed 10 yards in front of the line of scrimmage on first down (it will make sense in a minute). So if your team starts with the ball on the 10 yard line, then the line of scrimmage begins on the 10 yard line, and the line to gain begins on the 20 yard line. Your team has 4 chances ("downs" or plays) to advance the ball past the line to gain. If you manage to do so, you get a new set of downs (i.e., four more plays to advance the ball). The new line of scrimmage is placed wherever the ball ends up, and the new line to gain is placed 10 yards in front of that. (note: the line to gain only moves when your team gets a first down, but the line of scrimmage just identifies where the ball starts on that play).
If you fail to advance the ball to the line to gain after four plays, the other team gets to start wherever you left off. This is very important. If your team starts on the 10 yard line, and after four plays has not reached the 20 yard line, the other team takes over very close to their end zone and an opportunity to score 6+1* points. For that reason, on fourth down, teams will often choose to punt the ball as far away as possible (to put the opponents farther from the opportunity to score), or to kick a field goal (kick the ball through the yellow posts in the end zone). Punting only changes the position of the ball on the field; field goals score your team 3 points.
Rules summary Your team gets the ball and has four chances ("downs") to advance the ball at least 10 yards. If they succeed, their counter resets to 1 (it becomes first down again; you don't cumulatively gain extra plays). If they fail to get a first down in their first three plays, they usually (but not always) will elect to punt the ball far away or kick a field goal. Your team will try to advance the ball by running it forward, or by passing, but you can only pass the ball forward once per play, and only if you're behind the line of scrimmage.
There's a lot of other shit involved like penalties, special teams plays, safeties, and strategies, but you don't really need to understand them to enjoy watching a game, and you can just ask your friends to explain what a particular penalty means when it happens.
A touchdown technically only scores 6 points. After scoring a touchdown, teams have the opportunity to either kick a short field goal for one extra point, or attempt to advance the ball into the endzone again from a short distance for two extra points. The field goal is considered guaranteed points, while the *two point conversion typically is harder. Teams usually go for the field goal, although in certain situations and certain point differentials, the two point conversion is better.
4) So you want to play fantasy football. You poor, poor bastard. You can always subscribe to r/fantasyfootball, and have a really good shot at winning without ever knowing what the fuck is happening. Here are the basics: You and 7-11 of your friends form a league. At the beginning of the pro season, you draft players from the pro teams to be on your fantasy team. Briefly, you want to get all the best players, somewhat regardless of what team they are on. Each league is slightly different, but you will have a roster to fill. It