First
you’ll want to talk to your insurance agent about what it will cost to insure the make and model you are considering buying. You don’t want that figure to be a surprise, and you also want to find out how soon you will need to notify your insurer you have the new vehicle.
Second
and do this close to your planned purchase date — talk to your bank or credit union and get preapproved for the loan you’ll need. You may get something resembling a blank check (up to a certain maximum) that must be signed by you and the dealer. By getting preapproved, you will know the total loan amount and interest rate you qualify for. Even if you plan to finance at the dealer, it cannot hurt to come in with a preapproval.
Third,
make sure you have your driver’s license and proof of auto insurance with you. You shouldn’t be driving without these documents anyway. And, obvious as this seems, be sure you have a way of funding your down payment. If it’s not cash, make sure the dealer accepts the form of payment you’re planning to use. (If you forget to do this, you would not be the first, but that would be little consolation.)