I was wondering since I have always been in to bmx, is it normal for a new mtn bike to be hard to turn? Also, I get a little rub on my back break. I have mech. disc brakes. Also, there seems to be alot of chain slack, how might I be able to get rid of this?

It shouldn't be hard to turn, that might need some minor adjustment, but when you get a new bike, usually things need to be worn in and readjusted within the first month.

What I might adjust?

If you mean turn while riding then that could just be you needing to get used to the larger wheels requiering more effort.

If you mean lifting the front and turning the bars then the headset may be a bit to tight. The bars/fork should spin just as freely as they do on a BMX bike. If you just bought the bike take it to the sho and have them adjust it for you.

The rubbing of th brakes is normal with new pads. Once they wear down a bit it should go away although you still may have a very small amount of rub. If the rubbing is on and off as the wheel spins then something could be missaligned which should not be to hard to adjust.

There is a chance that the disc is warped but if it is a new bike then that is not very likely unless some shmo put it together.

DBD