When you have 2 jobs and a side gig it is hard to determine the exact taxes to be withheld. Both your employers will assume that this is the first job. So the taxes are withheld accordingly. You should go to the IRS website and do the tax withholding calculator, IRS Withholding Estimator . This will tell you whether you will owe or get a refund.
Based on this you have 2 options, if you owe taxes:
1) You can make quarterly estimated tax payments. These payments are due on April 15, June 15, September 15 and Jan 15 of next year. You can make these payments online at IRS Payments . Select Direct Pay with bank account or Pay by card. With Pay by card there is an additional fee charged by the IRS.
2) You can adjust your W4. Make sure you are taking the correct filing status and not claiming too many dependent as this will reduce the amount of taxes withheld. Then on the W4, Step 4(c) you can ask for additional taxes to be withheld.
Nisha