OK - I'll give it a shot...
For your back design, as I look at it, the first thing I think is lose the blue geometry around the red center design. I don't know if you should just go with what you have as the red right now, or if you should work up something new in place of the blue. I think the blue shapes take away from it and make it too chaotic. Your red design is pretty cool though. Possibly something with arcs and smooth geometry to break up the angled, chiseled, hard edges - can't really say without seeing it. For your courts, were you going for Harry Potter? If not, they need an overhaul. If so, I still think they need work. This is where you really need to study what's out there for really well designed custom courts. Here's a hint - the faces are always mysterious. Never give too much away. Also, Don is right, the proportion of the heads to the body is out of whack. And the hairstyles just don't do it for me. I would also advise using more muted tones in your color palette - the super bright reds and blues are way too much.
As far as software, Illustrator is the way to go, because vector paths are much smoother and precise than raster graphics. Plus, they can be resized ad infinitum without losing any quality. Photoshop is then used after your design is complete to add texture, filters, complex gradients, custom brushing, etc. Everything done in Illustrator can be redone, resized, and reconfigured easily without sacrificing anything - Photoshop, you're pretty much stuck with - short of hitting the "undo" command 100 times to get back where you were.
Basically I would agree with Don for step one: go out there and check out every good custom deck you can find. Study it. Look at the artwork. Look at the colors. Check out super successful projects on KS; and compare to the unfunded duds. I do this regularly - and can see a real defined difference between the two. Your graphic design skills for 15 years old are impressive. Study the craft, absorb everything you can. Emulate the good; purge the bad. One of the greatest graphic artists of all time, Paul Rand said "Don't try to be original, just try to be good".
Good luck.