can i just use my built in mouse in my chrome book
I wouldn't recommend using your trackpad. It might work but it makes for a pretty bad experience. You can use a controller if you have one, or just buy a cheap wired mouse.
@enkcĺark as @Babaganuche7 mentioned.
You can use your trackpad and it might work well with games like Steamworld or other more arcade-ish games.
If you wanna play Zombie Army or Destiniy for example, a mouse or controller would be a better solution.