Teams are the PvP (Player versus Player) aspect of Pokémon GO. While you can join your teams early on in the game, you won’t be doing any battling early on of course, at low levels, your Pokémon are weak and will need a lot of training to become gym-worthy Pokémon.
Once you reach level 5, the next time you tap on a Pokémon Gym, Professor Willow will ask you to pick and join a team and there are 3 choices: Team Mystic with Articuno as their Pokémon Avatar and Blanche as the Team Leader, Team Instinct has Zapdos as their Avatar and Spark as their leader and finaly, Team Valor, led by Candela with Moltres as the Team Avatar.
Be careful with this moment as your choice is permanent (barring asking Niantic to change your team). The only consolation you have is that there are no differences in the teams just that you’re choosing who are your allies and who will be your enemies. So go train your Pokémon more to help out whichever team you will choose.