How to fix this problem?
 

Im trying to bring my mountain bike back to life. I had to replace both gear shifter cables and not I am having trouble getting it to shift correctly. My bike has 3 gears in the front and 7 in the back. And I'm having trouble with both.
For the rear
My right shifter has 1 through 7 on it. 1 goes to the biggest sproket and 7 to the smallest. But when I shift to 2 it skips the second sproket and goes to the thrid sproket. And number 6 is the 7th sproket. I dont know how to explain it but hopefully someone can help.

Here's a tip:

1. Search on the bike forum for index rear derailleur adjustment for the basic concepts, which it sounds like you're trying.

2. Tip: On your particular setup, you start with shifter at top gear (7), and chain on the smallest cog, and RD cable slack. BUT, UNSCREW out the the tension adjuster barrel. 4 - 5 complete turns to extend the housing.

3. Tension and fix the RD cable to the derailleur, and then shift to 2nd smallest cog (speed 6 on the indexed shifter).

4. If it's like before, it should go to 3rd cog. Now screw in the tension adjuster barrel until the chain drops to 2nd cog.

5. Do final tightening of cable clamp bolt, adjust H-set screw, and then slowly shift from 7th to 1st gear. Adjust L-screw.

Done.

The problem you're encountering is that the 1st cog on an indexed RD system is often using the H-limit set screw as a stop and the cable should be secured only with fingers (without 4th hand tool), and the design counts on a known degree of slack in the cable initially because the shifter picks up more cable on the very first click.

Thanks But what are these things?
1. Tension adjuster barrel
2. cog
3. rd cable slack

Your rear cable is too tight. There is an adjustment where the the cable goes into the rear shifter that can be turned in or out. That is the barrel.

http://www.parktool.com/blog/repair-...nts-derailleur

1. Tension adjuster barrel (cf. Google the terms "derailleur tension adjuster barrel" and first link is to Park Tool website with video....)

2. Cog - synonym for gear or sprocket.

3. RD cable slack - != rd cable slack. RD in caps is an acronym for Rear Derailleur.

Good luck.