How do I find out how much I will have to pay in taxes for withdrawing my 401K and how much in penalties?

The penalty for an inservice withdrawal is 10%. The amount of the withdrawal is added to your taxable income. So you have to do your taxes to determine that part.

If you are under the age of 59 and 1/2 you will pay ordinary income tax on the amount withdrawn plus a 10% penalty for early withdrawal. If you are 59 and 1/2 or older you will just pay ordinary income tax on the amount withdrawn.