The first thing to consider is the condition of the drive train, specifically the cables and the derailleurs. Did you replace the cables? If not Did you remove them and lube them with oil or synthetic lube (NO grease). Next would be to make sure the derailleurs are lubed at the pivot point. (notice the body shaped like a paralellogram...Lube these points to make sure movement is not comprimised. It goes w/o saying but the chain should probably be lubed, preferably replaced.

Once cleaned and lubed the next thing to go is start by adjusting the rear deraileur. The 2 little screws are used for this. My starting point is the smallest cog position. Move the shift lever to the position that would normally position it in the smallest cog. It can also be a good idea to completely loosen the rear cable here. Now for the screws. The upper screw, Usually is the one that corresponds to the high gear, which is your smallest cog. What you need to do here (w/ a loose cable) is adjust the rear derailleur so that the top cog aligns with the small cog on your freewheel. For good measure you can move the derailleur back and forth with your hand to see if it returns freely to the same position and is aligned with the small cog when looking from the rear.

Next step: Connect the rear derailleur cable. Now the next thing you may want to do is check the adjustment on the other end (large cog). Spin the wheel and move the derailleur to the large cog with the shift lever. If you can't make it to the top, then loosen the lower screw on the derailleur. If it goes to far, or shifts off the rear cogs between the spokes, tighten the lower screw. (Be careful here. If it appears to be coming off and shifting between the spokes, stop and adjust the derailleur). Turn the 2 limiting screws only about 1/4 turn at a time when the upper and lower limits are close.

Now that the upper and lower limits are set you can work on adjusting the shifting. Shift to the small cog again. Now, while turning the pedals, shift up only one click. If the chain has trouble moving to the 2nd cog, then turn the rear adjusting barrel towards the direction the wheel (i.e. tighten the tension on the cable). If it goes up more than 1 cog with that first shift then loosen the tension by turning the adjusting barrel towards you. If the chain landed in the second gear smoothly, then shift again to see how it goes in to the next gear. Go up and down one click at a time. Use the adjusting barrel to fine tune the adjustment and make sure the shifting is smooth. Usually you can see, or hear, the chain if it's slightly offset on the cogs. A rule of thumb is to turn the adjusting barrel in the direction that you want the chain to move to.

If this doesn't work, there could be other problems like a missaligned derailleur hanger (the part of the frame that the derailleur is mounted to), or even a possibly bent derailleur. This is not abnormal, especially on a mountain bike. If all else fails or if you're not comfortable making the adjustments yourself, the local bike shop will be happy to assist.

Regards,
ken