Wicker Man

Eating is the key to gainning mass. First of all do not give up on trying to reach your weight goal. I had the same problem you had when I was your age. It will all come in due time just keep at it.
You are 14 so do not make the mistakes I use to, don't kill yourself trying to gain mass. Small guys need short intense workouts and alot of resting time.
Eat around every 2 1/2 hours, having 5 or 6 meals a day. Try to get good protein, Carbs and Cal's in these meals. Around a gram of protien for one pound on bodyweight.
When I bulk I keep track of my Calories and protien in a note book. If I'm not gainning weight I up my calories approx. 500 a week until I see results.
If you do alot of cardio, you should maybe slack off a little because you are using alot of you Cal's. When lifting weights focus on power exercises like Squats, Deadlifts, Clean and press and Bench Press. Squats and Deads are the king in weight lifting, but make sure you know how to do them correct. Good form is a must with these exercises as any you do. Do a search to find proper form (if you already don't know it).
There are lots of Routines and mass gainning ideas all over the internet,
just eat alot, workout hard (don't over do it) and don't give up. It will all come in due time.