Did you mean the difference between 'at' and 'in'?
As for have / has, use 'has' in the third person singular and 'have' in all the others.
You
have a nice car.
He
has a nice car.
Interrogative and negative (always use 'have'):
Do you
have a nice car?
Does he
have a nice car?
You don't
have a nice car.
He doesn't
have a nice car.
Interrogative and negative are different with 'have got':
You
have got a nice car. -
Have you got a nice car? - You
haven't got a nice car.
He
has got a nice car. -
Has he got a nice car? - He
hasn't got a nice car.
Hope it helps.