The interest rate shouldn't be your only consideration.  You can
get ridiculously low rates (like 0% - I haven't heard of anyone
negotiating rates lower than that, but I don't think it's inherently
impossible, but expect late fees) for certain cars with inflated
prices in a dealer-financing deal.  You still might be overpaying.
Negotiate the price, loan terms, car features, and warranty as one
combined package.

Put down as much money as you can afford, consistent with not having
to later borrow money on credit cards because you tied up too much
money in the car.  You'll save a lot on interest payments.  A bigger
down payment may help lower interest rates (it does on houses - not
sure about cars).  The best deal can be to negotiate a good price
and pay cash.

Check your credit report and clean up dings on it to the extent
possible.  Paying someone to do that for you is probably expensive
and likely to be a scam.  Nowadays your credit score is probably
better than the scores of the manufacturer and the dealer combined.
Don't be afraid to ask the dealer for a copy of *his* credit report
before putting down a deposit.

Yes, you should consider not using a loan at all.

Go for banks as the preferred method. I've heard endless complaints about
auto company financing - which *is* done by kickbacks BTW in case you
didn't know.

However it's far better just to save up 5 grand or so and get a decent
used car. It will last years if you properly check it out and maintain it.
In addition you won't lose any income on a monthly basis nor can it be
reposessed.

Join a credit union and check their rate.

Mine charges 5.50% for 24 to 48 months and 5.99% for 60 to 72 months.