You need to look up stuff online: how to replace shifter / derailleur cables, how to set up and adjust derailleurs. Plenty of how-to vids on YouTube, and the Park Tools Repair Help site is also extremely helpful.

If you just want to get it into an easier gear out back, you can effect that a couple different ways at the rear derailleur:
1) Cut a very short section of shift cable, maybe even one you took off the bike or still have taped out of the way, make sure it includes the end with the knob on it which is at the shifter, run it through the stop on the derailleur, position it about where you want it to be, and fasten it down. You can then use the adjuster where the knob is seated to fine tune the chain in relation to the gear.
2) adjust the hi limit screw on the rear derailleur as far in as it will go. Might be how it's even currently staying in the second gear right now...

You'll get more range of adjustment and probably lower gearing using method 1); method 2) is the simplest.