I HAVE A SCHWINN SIERRA MTN AND DO NOT KNOW HOW TO USE THE TWO SHIFTERS

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If the derailleurs move by hand OK and the shifters don't then you might consider the cable that connects the two - probably rusted out.

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Mtn bike shifters get dusty, at the shop when shifter didn't work properly we would wrap the shifter pod in a couple rags and spray WD-40 or Liquid Wrench in there to flush them out, a lot of it, then actuate the shifters and work through the gears, usually takes several times of flushing and working the gears to get them cleaned out. This works particularly well on older bikes that don't "click" when you move the lever. Also make sure you lubricate inside the cable housing and make sure the cables are not crossed in their routing and that there are no kinks or tight turns in the cable routes where the cable could bind up a little.

http://www.parktool.com/blog/repair-help/shift-levers-shifters

Keeping a derailleur in a given gear is the shifter's job. If the shift lever/mechanism doesn't click and stay where you place it, something is wrong internally. It will need to be replaced if, for example, a SRAM MRX grip slid away and let the guts fall out.

Whatever your particular dilemma, look for a friendly local bicycle shop to see what they can do for you. I replaced the entire MRX shifter set for a friend using an upgraded and better designed Shimano setup for under $25 and my own labor. But then, I'm an old hand at working on bikes.

For DIY:

http://www.parktool.com/blog/repair-help/categories/derailleur-systems

http://www.parktool.com/blog/repair-help/shift-levers-shifters

If, I were you I would get a tune up on it and then ask the mechanic how to shift. It is too complicated to go into here. You can try googling instructions, but the book that came with it, really did not go into detail. I know I have been a Schwinn dealer since 1979. You always have to be pedaling forward to shift, do not pedal backwards, you can chip teeth off of your chainrings.

OK, for a beginner in the world of 18 speed bikes, put the chain on the big sprocket up front, it should be about 46 teeth, make sure it is center and doesn't make noise, if it does, adjust it just a hair either way, now, the rear, put the chain on the big sprocket in the rear, make sure the front didn't start making noise, if it did, adjust it again. You are now ready to take off, in low gear, as you start out, it is easy to pedal, soon you will be peddling air, shift into second gear in the rear only, don't touch the front. Just a little push with the thumb will make it shift, now were going faster, easy peddling, shift into 3rd, then 4th, etc, coming to a stop, down shift the back sprocket back into low gear (big sprocket) and be ready to take off again. Soon you will be shifting that thing like a Ferrari! The front sprocket, will be used for moderate terrain, wheelies, and mountain climbing, that will come later, learn how to shift the rear first. As each gear climaxes, hit the shifter, it will get smoother as you go, and keep the chain quiet, no climbing on the next sprocket or falling out of gear. Hope this helps.

You may end up spending alot to rebuild the shifters, brakes, tires, saddle. Have you considered checking the local craigslist and picking up a reasonable priced replacement? You may come out ahead. If all the cables, shifters etc are rusted and having to be replaced, it can add up quick. Sorry to hear about your truck..

It sounds like the limit screws on the derailleur are out of adjustment.  If the derailleur cant pobbible move any farther towards the 3rd gear and the cable has slack in it then we know for a fact that the limit screws are to blame.  
If you can move the derailleur with your hand past the 3rd gear then we know the cable needs to be adjusted.  I would recommend taking it to a bike shop as those index shifters do take a bit of work to adjust correctly.