I'm 5'9", with a small frame. My "ideal" weight is supposed to be 144-148 according to the tables from 10 years ago.

To gain, you are going to have to eat more. To gain muscle, instead of fat, you are going to have to eat more, and do load bearing, non-aerobic exercise. Weight lifting, push-ups, ab curls, pull-ups, power yoga, rock climbing, and other things like that will build muscle.

Adding muscle while you're young will help you maintain a lower body fat when you get older. It's common for young skinny types to add body fat as they get older, since they start with less muscle. You lose muscle faster as you get older. This muscle burns calories, even while at rest, so when it's lost, the body doesn't burn all the fuel taken in. A good regimen of weight-bearing exercise will help avoid that trend.

Jonathan