Julia is the first leader(electric type) , you can defeat her by just leveling up your pokemons. You can get either a Zigzagoon or a Pachirisu. The former is great learns most of the HMs and also has good speed, Pachirisu is also a good option in the early game. You can also get the gulpin or trubbish. They will help against Florinia (the second leader, Grass type), your starter will also be helpful in this battle.
There are some event Pokemons like tynamo , mareep , Espurr (You should get it,helpful against Corey and Kiki.), Numel , Whismur and Petilil.
Mahurita or Pancham will a great backup against Florinia. You can also get a Kling( good against poison leader Corey).
Hope I was helpful.