Ignition Key won't Turn Please Help

Hi, When I put the ignition key in my 2001 Durango, I can't turn the key to start the car. The key seems to be stuck and won't turn. I can get it in and out of the ignition with no problem, and it WILL turn every once in a while to start the car. Any Ideas???
Thanks inadvance.

first get anew key made,might be worn . if that doesnt work the ignition lock cylinder needs replaced