I've been skinny all my life, in fact I'm considered underweight. I am around 5' 8'' and around 120 pounds. I can see my ribs and the thought of losing weight scares me. Will I sort of even out and gain weight on paleo?

That kind of leads to my second question; being really skinny without much muscle, what does it take to fill out? I've been to weight lifting forums where they follow very specific diets and do special programs with barbells, but joining a gym is way out of my budget right now, the diet part is already taken care of or I wouldn't be here. I know I won't get ripped from bodyweight exercises, but if I did them on a consistent basis ( pushups, planks, etc... ) would I see a difference at all? Or does it take a dedicated strength program?