In order to join a team, you'll have to go to the teams menu. There are different ways of joining a team.
Once you access the tab, you will find a search tool in which you will be able to enter the name or code of the team you wish to join.
You'll also see a list of teams looking for players and you'll be able to join the one you want. You will be able to see the team's name and the number of members it already has.
Bear in mind that each team has a maximum of 50 members, so you'll only be able to join those with free slots.