In the first gym you will face ground type pokemon. To defeat these type of pokemon you need to get grass and water type pokemon. The pokemon in this gym are about average level of 18 so you want to level your grass and water pokemon to about level 18. To find these type of pokemon you can go to lakeside for water pokemon, get barboach,then go get grass pokemon go to route 502 where you can get Lotad or Oddish. Once you have your pokemon (maybe 2 water 2 grass) go to route 514 where you should face sandshrew who is a ground type pokemon, just like you will face in the 1st gym. Once you have reached level 18 on all your pokemon you should be able to defeat the 1st gym. Remember to only use grass moves and/or water moves as they are super effective.
how to get to 1st gymEdit
1. Go to Battle tab and go to Gyms
2. Pick top left, Gym leader Sandrine, avg level 18, prize $500-10TP
Google images--pokemon effectiveness chart