How do I find out how much is in my 401k and how do I cash it out?

You should be getting regular statements, but you can check online if your company provides online access. You can write them and ask for a balance.

You do know, depending on age, that you will have to pay full taxes and a 10 percent penalty to withdraw.
Also are you still working for the company that provides it, then often you can not cash it in.

Is this with your current employer? Have you talked to the plan administrator?