>>2459501
you should start by telling us which country/state and by not asking this on 4chan

>>2459501
fucking google it shit head

All income is taxable, just list it as "Other income" when you file your taxes next year. Look up your local taxes. If you live in NYC for instance, you'll have to pay both state and city sales tax. However if it's all in cash and your art sales aren't a regular thing, you might consider it an under the table sort of thing.

There are loads of invoice templates online, just use google.

Since this is a sizeable amount of money, you may want to consider having them put down a deposit and sign a contract that basically says if you don't fulfill your end, the deposit is refunded, if he tries to back out before you finish you keep the deposit, and if you complete the work, he is legally obliged to pay you. There are some freelancer contract templates out there too, just do your research.

You should have local places to call for such help

http://www.artpact.com/Contracts

Generally a decent resource for how to draw up legal shit for commissions and such.