Problems shifting my mountain bike gears

[cyclejohn]

My 2nd 3rd and 4th gears all work but it won't shift into 1st 5th or 6th. Even after my gear cables and shifters were replaced with new ones.

 

[davereo]

Did you take a small screwdriver to the two small screws on your rear derailuer? One problem could be that your stops are over adjusted and preventing your deraileur from moving the entire distance.
Another problem could be that your new cable is too short preventing your derailuer from shifting into 5th and 6th.

Check out this site they have some basic tutorial videos that should help you out fixing your bike.
Good Luck
http://bicycletutor.com/