Serpentine Belt Coming off?
 

I have a 1990 f-150 V-8 5.0. Lately the serptine belt has been sliping out of alignment on the AC compressor pully. it doesnt totaly come off but about half way.

Is there a way to prevent that or tighten the tensioning spring?

You have a pulley out of whack somewhere. Did this just start after you replaced an accessory? It could be the tensioner going bad, too.

ya i just replaced the alternator. but when ever i pull back on the tensioner its really easy to pull on it. I usually have to use two hands and walk around to the side of the truck. Lately i can do it with one hand.

Sounds like the tensioner is going bad.They are not to expensive and that should take care of the problem.

Check pulleys as well
 

A new tensioner is a good start but Check Your pulleys as well.
I had a issue like this on my F-150 (4.9 I-6).When I replaced the tensioner my belt was still coming off due to the water pump trying to freeze up.

how much would a tensioner run for? (im a teenager with very little money)