Well it depends if you want to be as true to the anime as possible, if so then you should definately keep Pikachu, Squirtle, Bulbasaur and Charmander in your team, anyways Squirtle, Bulbasaur and Charmander are all good pokemon so I'd use them anyways.
Then it depends on what you want to have as your 5th and 6th type, you could throw in a flying type such as Fearow or Pidgeot. Just whichever you want really, say what types you want and we can probably help you out more.
