All four tires must be of the same make/model, and must be within 2/32" of each other, or else you risk drivetrain damage.
You have a few options, both with their pros/cons:
1) Buy a replacement tire that's shaved to spec and have it mounted/balanced
2) Buy four replacement tires
Option 1: You're buying a new tire and then taking life away by shaving it down. For someone on a budget, this is the way to go as it will be like $25-30 to shave the tire in addition to the cost of the tire and whatever mount/balance fees your shop charges.
Option 2: You get all new rubber, but that's a significant expense, and you're left with three working tires that you have to deal with. You can sell them to recoup some of the costs, but you're going to take a financial hit. You'll need to pay for all four wheels to be mounted/balanced.
See what tread depth you have left on your other three tires, and make a financial decision which best suits you. If your tires are relatively new, you're probably better off with Option 1. If the tires are nearing the end of their life, you might be better off with Option 2. Option 2 also gives you the opportunity to try a different tread pattern / compound for different driving characteristics (e.g., you want a more aggressive tire for better traction at the limit or a more subdued tire for better gas mileage and tire longevity).