I guess you just go with your gut.
Usually it is a team that plays from your city or state. Sometimes you will get acquainted with the team through your dad, mom, brother, etc. Sometimes you will see playoffs on TV and get to know some of the team players this way.
Maybe something as silly as the colors will make you decide to choose a favorite team.
Some people love to be on the side of the underdog (the team that is supposed to have the hardest time winning).
Remember, just go with your gut.
Hope this helps.....