Q: What do you do when a client pulls back on a deal and you've already sent them the files? Is there a way to protect the work you did for them from being used?
A: It's hard, unless they signed a contract then I don't think there's much you can do except be firm and patient regarding payment. It's an unfortunate lesson learned. Next time get them to sign a contract that protects you and never send the final files until you are paid in full.
Q: "Ok thank you. Now, how do you write contracts? Sorry I'm new at all of this."
A: I would ask an employment lawyer. Although it's expensive (around $250-$400) it's an investment for the future. Maybe you can find someone you know locally and offer to do some design work in exchange for a contract?
The other lesson to learn is knowing when this client isn't someone you want to work with. If you see any red flags that make you feel uncomfortable in the beginning, politely decline to work with them and direct them someone appropriate like fiverr