problem in gear shifting I'm using a  21 speed bicycle,the problem is I'm unable to shift the chain to the smallest derailleur(rear)...the shifters are thumb press type
there's no problem in pressing the shifter...when shifting to the toughest gear the chain shifts up to second smallest derailleur(rear)..when i press the shifter it's moving but the chain is unable to catch the smallest one...can't figure out the problem :(
It sounds like you need to adjust your rear derailleur - the cable may have slipped a bit and have insufficient cable tension, or if you recently had a crash, you may have bent your derailleur. 

I'd recommend reading this to learn how to adjust your derailleur.