just bought nishiki pueblo came stock with some crappy sunrace shifter and after about 300 miles of riding the shifter has jammed i can't get the gri shift to turn . im new to the scene so take it easy on me.
Are these grip-shift shifters? If so, I pity you. I swear some of those grip shifters are designed to be disposable.
Assuming it hasn't been left in the rain, make sure nothing has jammed the cable or derailleur. Dribble good chain oil, like Tri-Flow, into the shifter and try to free it up. If that doesn't help, you'll have to disassemble the shifter, and there is a good chance you won't be able to reassemble a grip shifter.