In Toon Blast, you have to beat level 20 to be able to join or create a team.
Once you are level 20, you can easily join a team by using the "Team" tab! From there, you will find teams to join, or you can use the search button to find and join a team.
The "Join" button can be used to join a team if there are positions available. If a team is "closed", you will need to wait for approval from the Team Leader before you can join!