Choosing a team in Pokemon Go is extremely important, and can even ruin friendships if you end up on the wrong one than your friends.
Once you’ve reached trainer level 5, you will need to walk up to and click on a gym. In doing so you’ll be asked to choose a team to join.
Choosing a team means you’re forever bound to uphold the reputation of you chosen color. As an example I chose Team Mystic, and so did 90% of my friends. Now we like to get together some nights after work and go around taking all the gyms in our town and showing everyone Team Mystic is boss around these parts.
No matter what team you choose just remember don’t Pokemon and drive and always be aware of your surroundings.