To be coach in Soccer-1, you have to be at least 18 years old.
After you register, if you're old enough, Soccer-1 will ask you if you want to be a coach, a player, or both. Don't worry if you make the "wrong" decision now, you can always change it later.
If you pick coach (or coach and player), you will be able to set up your coaching profile. Your coach profile is visible to other users in the system, and helps them decide if you are the right person to coach their group. Your coach profile has what we call a "home base," that's where you operate out of. Group owners can use find you on the map and look at your profile.
If you are a player, and want to be a coach, go to your Profile (you can get there from the person icon in the navigation bar). If you are old enough to be a coach, you will see a button labeled "Choose Role." Hit that and you can change your role at any time.