How do you adjust your disc brakes on mountain bike?

i just recently bought this mountain bike and it is working good but the disc brakes need a bit of adjusting i think. this is the first time i got disc brake bikes and don't know much about them. the problem is that the brake pads have contact with the disc or the rotor when i spin the wheel. it doesn't spin freely and spin slow. also i checked that the disc is a bit to the left in the caliper. i tried to loosen but it didn't even move. can you guys help me or possibly give me a guide?

thankyou.

If the bike you just bought is from a bike shop take it back to them and have them fix it. If it is used it depends on what brakes you have. The calipers need to be centered.
Avid BB7, you can use the red knobs to adjust the pads if the centering is not too bad.
On hyd. the calipers need to be centered, unfortunately you might not have the spacers or other related parts to do this, in this case I would recommend taking it to a bike shop where they should have no issues making the adjustment.

I dont know about bikes, but on cars new pads have to be broken in. That way you do it is stop the car from a pretty fast speed in a steady manner. Your pads may be glazed. If you can get to them, try to rough them up with sandpaper and then try it. Sounds like them are tight enough against the caliper.

Adjustment procedures differ for different models and manufacturers.