Reddit mentions of The Art of Mixing (Mix Pro Audio Series)

Not bad for 14 at all. What really is going to improve the quality of your beats is mixing. A couple quick tips.

Put Fruity Compression on the kick and snare (If you need help with assigning plugins to channels, I can help.) Hit the preset arrow left once and the preset should be called "vintage" (it has been a while)

Experiment with compression on the piano too. This not only makes things a bit louder and crisper, it also creates a plane for dynamics, as it stands everything sits in the mix at the same level and it's boring to the ear, aside from your melodic ideas.

If you've got a few bucks to spend, check out this book. I read it when I was like 16-17 and it helped create a nice foundation for learning how to mix. I used FL for a long time, and it's a great program. If you ever have any questions, holler at me.

Experience. Get a DAW and some plugins. I reccomend cubase or ableton live. Massive, sylenth, and z3ta+ are all good if you want to learn how to design sounds.

Read this to get you started

http://www.amazon.com/The-Art-Mixing-Audio-Series/dp/0918371171

this too, it's free

http://noisesculpture.com/how-to-make-a-noise-a-comprehensive-guide-to-synthesizer-programming

It requires a lot of time and dedication to learn to produce something that doesn't suck so keep that in mind when your first few projects ineveitably end up sucking. Just keep making progress, don't get discouraged, ya know?

This book covers the subject admirably.

http://www.amazon.co.uk/Art-Mixing-Mix-Pro-Audio/dp/0918371171