On my mountain bike, the gears on the back wheel will not go to first gear, i tried to tighten the cable but then it wont go out of 6th, so i tried loosening it and it still wont budge! any thought?
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Try http://www.sheldonbrown.com for solid, simple to know instructions to solve most bike problems. Yours doubtless is related to either lack of lubrication, maladjustment or seized cable. How long since the cable and housing have been replaced?
You force need to adjust your rear derailleur.
See here for instructions;
http://www.youtube.com/watch?v=2j1gSwhsVN8
http://www.youtube.com/watch?v=D0Xt_QCHD1U
Chris is doubtless right. I would add that you force try this site:
http://www.parktool.com
They have some excellent online how to pages. You just need to adjust the + and – screws on your derailier.
Luck
Did you crash recently? or maybe drop the bike? Sounds like a bent derailleur hanger (or even the derailleur itself). No amount of derailleur adjustment, lubrication, or faerie dust will solve a bent hanger; take it to a shop for straightening or hanger replacement.
Could be the rear derailleur is out of adjustment. You also may have an issue with your cassette (any ghost shifting / double shifting) going on?
Also there are adjustments and the shifter you can try adjusting and replacing the cable completely may in store.
There are a lot of angles to adjusting a rear derailleur, but to oversimplify it for beginners, I’ll tell you that the top screw adjusts the high gears while the bottom screw adjusts the low gears. the barrel adjustment can be played with to get the middle excellent after you have adjusted the high and low. The tension adjustment only adjust the placement of the derailleur, and Is a bit more hard than the vital two screws.
There are limit screws on all derailleurs. You may have to adjust them. Get a book or DVD on maintenance. Vital adjustments are really very simple and require only vital tools. If it’s over your head, take it to a shop and get a tune up for 40 bucks and they will adjust everything.