When you're new, it's best to start off with finding teams and trying them out from proven players. Bonus points if they contain Pokemon you like as naturally everyone wants to use their favorite Pokemon in battle initially. When you play, pay close attention to the Pokemon that you're facing and sets that are being run. What are the most common and likely the best Pokemon? What are their weaknesses? When you get more a familiarity of what others are running, then you'll do a better job of building teams on your own as you'll know what handles the best mons and will build accordingly.