I am a former college basketball player and current basketball skill trainer. (http:www.theworldofhoops.com)
Ball handling requires a lot of repetition. Especially as a big guy you are going to need to put the work in. But MAN!!!! At 6'5"....if you get your handle right and your shot right (I also answered on your 3 point post) IMAGINE THE POSSIBILITIES FOR YOU! If you make up your mind to be great, and are willing to put the work in...the potential is HUGE!
A good idea is to continually vary your ball-handling drills, and continually learn new ones. This requires you to adapt and will better serve your handle in a game situation.
Check out the School of Hoops ball handling section (http://theworldofhoops.com/school-of-hoops/ball-handling)
Again, it's hard to tell you which drills to work on without knowing how good your handle currently is.
I recommend starting with the one-ball dribbling drills (figure 8, around one leg, killer cross, etc)
Then move on to the two ball dribbling - be patient, these can be very difficult to master
Then a little more advanced (but some of my favorite drills) are drills with tennis balls
If you are already 15 and you need a lot of work on your handle, you really need to get serious...you should be working it 4-5 days per week.
If you have any questions or you are interested in a customized training program, feel free to contact me at joe@theworldofhoops.com
Good luck!
