Anyone can gain weight with excercise and proper nutrition.
The first thing you need to do is make sure your body has everything that it needs. I reccomend taking protein supliments, a high quality multivitamin, ionic trace minerals, vitamin e, fish oil, 5-htp(tryptophan).
An enzyme with a decent amount of protease can boost protein uptake by 300%.
Vitamin e, fish oil, 5-htp(tryptophan) can also replace prozac in most individuals since they will produce elevated seratonin levels in the brain, leading to an overall increase in hapiness.
After a month or so you should now be addicted to the way you feel on these supliments.
This brand is good for "multivitamin and minerals"and "green phyto foods"
their "super antioxidant" product is also very good.
Edit
for protein I recommend a decent quality ionized whey protein isolate.
"designer whey" is one of the best, and their unflavored variety has an excelent taste. Mixed with rice milk provides decent taste and better absorption. Take with enzymes to aid in digestion. Avoid mixing protein and milk, it can lead to signifigant digestion problems.
The diference between excellent nutrition and marginal nutrition is weather or not you want to see signifigant results in months or years.