Many people who work two jobs get a rude awakening at the end of the year when they calculate their taxes and discover that they owe some huge amount of money they no longer have. Again, this happens because of simplistic IRS formulas. Each company you work for will withhold for you as if that is the only money you are going to earn all year. To make sure you don't come up a couple thousand dollars short, you should change the number of exemptions you claim. An exemption is essentially a discount on your tax bill.
Every unmarried person gets one exemption as a matter of course.
Your employer keeps your W-4 form on file. It is a simple matter to fill out a new one. The rabies vaccine thing was just a minor revelation compared to this. You have to have health insurance. Don't even think about not having it.