You can look at the 'Best teams' page (under Achievements) and check some team pages. If they are advertising for new members they usually put it on their page. If som there will also be a note about who to contact if you want to apply for a place in the team.
Sometimes teams have certain criteria, like having a number of aging points and being able to keep that up by having a lot of horses, doing a number of blups per week/month etcetera, or they acquire blupping experience and/or game experience. There are also beginner-friendly teams who are willing to teach you the ropes.
If you see one you like and that is recruiting, send an application in which you tell them what you can bring to the team.
You can also put on your personal page that you are looking to join a team. Someone might see it and contact you.