Does the wheel rub the same amount for the entire circumference of the rim, or does it only rub in one spot?

If it is the former, the wheel just needs to be centered by loosening one of the axle nuts and centering the rim/tire between the frame tubes (known as the chain stays). Typically, you would loosen the left axle nut, because the right one is butted up against a piece that puts it in a fixed position, while the left can move in a slot (the piece of the frame is called a dropout, and your bike has slotted dropouts). But before you make the adjustment to the left nut, make sure the right one is as far back as possible.

EDIT: If the bike has a quick release, it may need to be adjusted tighter so it will keep the wheel from shifting. You still adjust the wheel alignment by sight using the same technique as a bolt-on wheel.

If it is the latter, the wheel rim is bent. It may be able to be trued, depending on the severity of the deflection.