hello I'm still new to riding and just picked up a new bike and i had to remove the back wheel, when i put it back the chain rinds on the cassette it seems to be not centered, how do i fix this?

Sounds like it isn't centered... loosen the quick release while the bike is on the ground (weight on the wheels) and make sure that the axle is seated properly... double check that the space between either seat-stay and the rim are equal on both sides. Tighten the quick release.

It's also possible that the brakes are interfering with your alignment when you are reinstalling the wheel. Loosen them if need be. It could also be a bent derailleur-hanger.

A better description of the problem, including pictures would help a lot.

You need to buy a repair book like the Zinn. Also look at the Park Tool site.

Take it to your local bike shop and have them show you how to remove and reinstall the wheel.

it will take less than 5 minutes and should be free.

this is a safety issue; don't mess around.

The wheel seems to be centered, and I have the gear set on the smallest gear, and when I pedal or back pedal the chain seems to grind on the gear next to it. I tried everything. What does it sound like?