Forgive me if this is a very "stupid" question to some of you; I'm very new with taxes, so I'm having trouble understanding how to calculate my taxes.

Basically, if I were to make $4000 in two months and stopped, how much of it do I have to pay in taxes? I live in Washington, so I do not have income tax, but I'm still subject to federal income tax. I just need clarification on how to calculate it. And do I also get tax returns?