Are they taking out Social Security and Medicare taxes? If not they might be paying you as a 1099 independent contractor instead of an employee. Better ask the emloyer. If you are an employee that will get a W2 you need to give them a new W4 form to have withholding taken out.
https://www.irs.gov/pub/irs-pdf/fw4.pdf
Either way if you are an employee or 1099 you should probably send in an estimated payment to cover the tax. Or you can have more withheld from your full time job. Although it's already December so you might not be able to take more out of your first job to cover the tax.
Here are the blank Estimates and instructions…..
https://www.irs.gov/pub/irs-pdf/f1040es.pdf
Or you can pay on the IRS website. Be sure to pick 2025 1040ES payment
https://www.irs.gov/payments
