Attempts to gain weight-How simple is it?

I've tried the health food stores' milkshakes and weight gainers-nothing helped. I neve r put on one pound. Okay, now somebody told me that I will gain weight in the future, probably when I'm in my late 20's. That's dreadful to me. I want to gain those pounds 2 weeks ago! Really, I never stuck with one protein shake or weight gain supplement or whatever names they're called. Some were even pukey! But I always gave up quickly. Next time I'll tell you the story of how I put on 10 or 15 pounds (can't remember for sure) and then lost it within weeks!