5 Gears usable out of 6 I have mountain bike which is around 1.5 years old. When I bought it, I used it without any hassles and all was fine. But one day I overshifted the friction shifter and the shifter was bent. Now it won't shift to chain levels. I knew that friction was of no use and I didn't look into this matter. A year passed, and now I am wondering about that issue because one of my friend bought a new cycle and he shows me, and I get burned up. 

The main issue is that when I brought the cycle I didn't knew so much about gears. And when I shift gears to the 6th gear, it won't shift at all. I have a 18 speed bike, and out of them I can only use 15 gears. When I shift to 1st gear, one gear remains to be shifted. This was the problem since I bought the bike. But now I am jealous of my friend. And, my friction shifter is out of order. It is bent and not properly adjusted. When I shift to 1st gear the chain falls down, and the shifter rubs with the chain. So I can only shift between 5-2 gears. Please help me, I am not so expert in gears.
A bike 18 months old is very unlikely to have a friction shifter.  Did you mean its an older bike that you bought 18 months ago?

Raise the bike off the ground, either in a stand or hang it from a tree branch or rafters by rope.  Then slowly pedal with your hand while pulling on the inner gear wire whereever you can get to it (front downtube is normal)

You should see the Rear Derailleur move through its whole arc of motion and the gear change should happen.  This would isolate the problem to your shifter's mechanism.

If the gear change doesn't happen, the problem is at the back. 

Consider posting some photos once you've had a closer look.