I have a full-time job with tax already deducted and now starting a part-time job that I will have to pay taxes on. How do I calculate my estimated tax payments?

Every calculator I try keeps asking for my income info from my full-time and figuring that in. But I'm already paying those taxes. I just need to figure how much tax to pay on my new part-time. I expect to make about $30,000 from it.