Well, it doesn't matter what Pokemon I'm using. What ever starter you pick, you alway stick with it. For me, I alway pick what my favorite starter such as Grass Type. Then I need to catch a second Pokemon to beat gym lead easy or a double battle. I use a flying type to advoide from weaking my starter pokemon like my Grass type against his Fire type. I use my third pokemon as my favorite such as Flygon to trained my favorite. I also use Exp. Share to level up my weak pokemon to get stronger. Until Ifinishes to whole game, I'm still training mu pokemon until 50 or more to build mu team.
Build a team:
A team like your Pokemon is a part of a group, but you need strategy to bea your opponent easy. Here the list?
Element team:
Well one of each of your Pokemon should be in one type.
Grass type
fire type
Water type
Electric type
Dragon type
Ground/rock type
Your team need to be in different type and it will be easy to beat yor team.
Full type team:
A full type is lie using all water team or something. It will not be easy to match your team because your opponent pokemon move know what type they against easy. Like Gyaradis learned flamthrower or Sceptile learned earthquake.
You don't have to do this, it your own Pokemon, but be careful. You will be beaten easily.