I disagree with doing something primarily to become popular. You should do something because you enjoy it. If it makes you popular, that's just an added bonus.
That being said, I'm an athlete so I can probably help with your dilemma. The first thing you need to do is figure out what position you want to play in each sport. For basketball it's pretty simple. If you're tall (6 foot+ range) your best bet is center. If you're short, (below 5' 10"), probably your only hope is point guard. If you're in between, probably a forward (if your school uses them). Once you figure that out you can start getting ready.
A point guard's job is primarily to pass the ball to the real shooters (either to a forward for an outside shot, or to the center for a lay-up). You need to keep your eyes on everybody on the court while dribbling the ball and be able to make good passes, while also having at least a moderate ability to shoot the ball. To practice being a point guard, I would suggest dribbling drills (one handed dribbling, switching hands, eventually learning how to spin round, etc.), mixed in with passing drills (get a friend and do chest passes as well as bounce passes and go over in your mind when you would use each of them). Take a few shots (especially three point shots), but don't be overly concerned with that. Mostly focus on being a good passer and a leader. You need good ball control to be a successful point guard.
For playing center, really your main job is to be big. You won't be taking many shots from further than about 2 feet from the basket. For forward you really just need to practice your lay-ups and focus on your defense, because that's the center's main job. They count on you to block shots in the paint (the painted area near the basket), and they count on you to overpower the other players. That means you need to be strong. Also, get used to the method of catching the ball from the point guard, taking one dribble TOWARDS the basket, and then jumping towards the basket to score. That's your bread and butter.
For playing forward, really, just practice your shooting. ESPECIALLY the three point shots. You are their go-to-guy when they need points. You really shouldn't be passing unless you can not get a clean shot off. Dribbling is still fairly important, since if you can't get a good shot off from the key or the three point line, they'll expect you to drive to the basket (dribble towards the basket strongly and quickly to set yourself up for a better shot.
What to work on for ALL positions: defense and running. Defense is important at every position. Keep your arms out and stay low. You should be trying to swat at the ball with one hand while trying to block potential passes with the other. Also, no matter what position you play, HUSTLE is key. After you score or the other team (or your team) gets the ball, you need to HUSTLE down to the other side to set up. The only exception for this is when the point guard dribbles the ball down on offense. Only then will you be taking your time because you need to let your team set up. That should be enough t get you by, your coach can go over the finer points of the game.
Now football... that's a completely different beast. Football has a LOT of possible positions to play. I won't go over what to practice until I get some specifics on you. I need to know if you're big or small (both tall and strong/fat), if you're fast or slow, if you're god at catching, how smart you are, how fats you think on your feet, and if you are good at throwing and/or catching. Also if you have strong legs, that helps at a lot of positions, but also has a couple positions all to itself (punter, kicker).
I will tell you this, though. No matter where you play you NEED to be strong, and you also need to be able to run for extended periods of time at most positions.
I must say, now is NOT the time to be learning a sport. Chances are pretty slim you'll make a high school team with no prior experience unless they are awful, in which case I don't know why you want to join the teams anyways. You really shouldn't even bother unless you genuinely enjoy the sports.