How can I GAIN weight?

I know this question might annoy some of you - but I am being serious.

I've been skinny my whole life, but until recently it wasn't 'too skinny'. I am almost 5'5 and from grade 8 - 12 I weighed about 112-115 pounds.

I am not anorexic, and I eat a healthy amount of food (which I prepare myself because I like to cook), but for the last 1/4 of grade 12 till now (I've been out of school since June), I have been unable to get my weight above 104. I look gross thin, and no matter what I try I can't gain weight. I miss having boobs, and not being able to feel every single rib!

I try to exercise at least a little every day (go for a walk, sometimes jog a few times around a field close to here), although I mostly stay at home (not attending University yet, so I cook and clean, and sit on my bum a lot).

I eat breakfast everyday (usually is either; omelette's, hash-browns, cereal, yogurt, or waffles).

My lunch is usually left overs from dinner the night before.

Dinner (homemade pizza, pasta, pasta salad, stir fry, etc) always accompanied by a salad.

I love to bake so I usually make desert as well which is cheesecake or apple upside-down cake, sometimes with ice cream.

I have tea at night and occassionaly a small cup of coffee in the morning.

I just don't get what I can change to start gaining weight. I usually eat until I'm overly full (which has become less than I used to be able to eat). Oh and prior to this year I mostly ate microwavable food and lots of chocolate (although I still eat some) and I drank a lot more coffee. My apetite seems to have gotten smaller.. but I think I eat enough fatty things to be gaining weight anyway (cheese, yogurt, dessert, milk, etc), and I always try to stuff myself.