My car was repo. and what happens when i can't pay the difference. can i make payments instead paying it all back at once?

You can negotiate payments, but I always recommend saving at least 30% percent of the remaining balance and negitiating a settlement. Get everything in writing before making your negotiated final payment. Then request a letter stating that the account is now "Paid in Full", instead of "Settled for less". Very important that you request this, so that they do not sell the remaining balance to a third party collection agency.