What about buying a car?

I am starting the process of buying a car. I currently 18 years of age and will be 19 in September. Can I sign a contract for buying a car or do I need a co-signer such as my dad? If not, do I need proof of a W-2 form saying I have money to afford the payments? Currently I am a Nanny part-time and make good money. Any help please!