Pokemon-go – How to pick a team

There are no fights before player level five. You will also not receive Potions and Revive items from Pokéstops before this level, as you have no use for them without access to fights.

Evolving can be helpful, although Combat Power (CP) is generally a much stronger indicator of success in a Gym battle. Additionally, you will want to build your team to combat the enemies' strengths and weaknesses. This follows the typical format from earlier Pokémon games (e.g. Fire is super effective against Grass, Electric is super effective against Flying, Water is super effective against Fire, etc.). This web page should be a useful resource in helping you build a strong team to counter your opponents' types.

The gym system is a little complicated, but in essence, by fighting an enemy gym you reduce that gym's prestige (until eventually your team takes it over). Once your team takes over a gym, you can increase its prestige by continuing to battle there. If there is an empty slot in the gym, you should be able to station a Pokémon there, which will help defend it from enemy teams. Once every 21 hours you can claim a reward for holding a gym by going into the shop menu and tapping the shield icon in the top right. The reward for each Pokémon that is still stationed in a gym is 500 Pokédust and 10 Pokécoins.

You actually have to beat atleast 1 pokemon in the gym to get some prestige. If you can't beat any of them then you won't get any prestige at that gym.