1. What sports are you good at or do you enjoy? Joining a sports team solely to make friends is fine, but it is much easier to do so if you are good enough to play and contribute to the team. So, the first thing you have to do is decide what you want to play, softball, volleyball, soccer, what?
2. What kind of time do you have to devote to this team? While it is a great platform for making friends, you are going to make enemies if your team is counting on you to show up for games and you can't, or if you only show up for a few of the practices. Base your decision as to what kind of team you will join on how much time you have to devote to a team. For example, a city league team may only meet once a week for either a practice or a game. A work sponsored league might have a couple practices a week plus games. A church team may only have games. A school team may have practice daily. An intramural type team may have a couple of practices a week, and a game. There are plenty of different options, so it is up to you to determine which best fit your time availability.
3. What gender of friends do you want to make. Usually when you make friends through sports they will be of the same gender as you, as the majority of teams are unisex. However, you can join a co-ed team if you are looking to meet people of both genders. Most schools and church run sports teams are unisex, but most city league type teams offer the co-ed option.

Once you choose a team the rest is up to you. You have to show up, and you have to not sit quietly on the bench, but strike up conversations, and try to get a good team spirit going. In addition to the practices and games, it may be helpful to suggest your team gets together for a BBQ after a game, or goes out for beers after a practice, etc. If you want to make friends you have to get together with people. So, look for opportunities to spend more time with people and get to know them better. Friendships will form, and you will find that win or lose the games you have a good time.