Bicycle Tutor

I got an '78 Schwinn, and I while ago I had a loose rear shifter selector, now I have a new problem, now the front gear will not shift out of the smaller gear now, it looks like the mechanism that moves the gear itself isn't moving over enough to shift it up to the larger gear. So how do I get the mechanism to move over enough? It use to shift over great before, now I got this problem and I don't want to take it in and pay the money.

Using your fingers- get hold of the bare cable that goes down to the shifter and pull it sideways to see if it moves the chain over.
Has the cable stretched or come loose from the shifter?
It may just need adjustment to tighten it up.
Sheldon Browns explanation on derailleur adjustments:-
http://www.sheldonbrown.com/derailer-adjustment.html
Scroll down to "Front gear"