sometimes when i am just riding without using the brakes the brake pads will rub on a certain part of the tire how can i adjust it

The main problem here is that your wheel is "out of true" -- ie., bumpy in places, lopsided, wobbly. Best thing to do is take it to your Local Bike Shop (LBS) and have them true the wheel. With the right spoke wrench and some know-how, it is possible to do this yourself, but it is among the trickier of repairs; there's an art to truing a wheel.

If the wheel is only a tiny bit out of true, and the brake pads are VERY close to the rim, you can just loosen the brakes a very little bit and avoid the rubbing without losing braking power. You definitely wouldn't want to do this more than a smidgen, so if a smidgen doesn't help, then get that wheel fixed.

How do you loosen the brakes? Some brakes actually have a handy little doohicky for making these minor adjustments. Otherwise, you would need to loosen the brake cable a fraction: first loosen the screw or hex-screw that clamps the cable in place, secondly let the cable out a few millimeters, third retighten the screw to secure the clamp. Make sure this is tight enough that the cable won't slip! That would be bad!