How do I figure out how much taxes to take out of a paycheck?

I'm a massage therapist and independent contractor, so taxes aren't taken out of my paycheck every payday. I'd like to start doing it myself, that way when tax season comes around I have it all put aside. How do I find out how much I need to take out of my paychecks? I'm married filing separately, if that helps at all. Thanks a bunch!