Do you use the to be verb in front of prepositions? For example, how would you say, 'I am at home' or 'he is under the car"?

It depends.
Prepositions at and under, in Creole, are used differently.

I am at home.
Mwen lakay mwen.
or 
Mwen lakay.

He's under the car.
Li anba machin nan.

Check out the posts on some prepositions.  Scroll down the list to the right side of your computer screen and find some posts on the prepositions at, in, on, to, from, etc...
Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words