Problems Shifting Need Help

[T-WIG951]

My rear derailleur system is off a bit. I have a shimano 24 speed rapid fire system. When im in first position my chain is on the second sprocket, then when i shift to second my chain is on the fourth sprocket. How do I fix this problem?

[trevgbb]

Dis-connect the cable from the RD, putting tension on the wire by hand, operate the shifter to ensure it is working properly and the cable isn't snagged any where and moves freely, fit a new one if in doubt.
Set the inner and outer stops on the RD by hand, get some one to hold the rear wheel of the ground and rotate the pedals slowly as you push the RD in and out.
Re-connect the cable, making sure the shifter is set to the right gear, and fine tune using the barrel adjuster.