How to buy a car?

I’m a little confused as how to buy one. If I have the down payment already, would I need to finance through the dealership? Or would I take a loan out from the bank? Would I only take a loan out for how much is leftover AFTER the down payment ? I’m looking at some cars that are around $12-13k. Any advice is welcome. Also how much do y’all think the apr is going to be if my credit score is 651 and it’s going to be my first car? Do y’all think I should wait until my score is higher?

Edit: I know the ones who already commented won’t see this edit but one more question. My TransUnion score is 651 but how do I build the other two or check it? Right now I’m with capitol one for my credit card but I use Whitney for my checking account