Reply to How to actually start being a DJ ?

So I'm new to the game. I'm a musician looking to jump in. I have the gear , I have the software, but where do I actually start. So I guess you might as well tell me how you all started. Right now I'm just kind of mixing and mashing music I already had in my Itunes library but I want to know how to build an effective collection of music that I will use and what is a good way to go about organizing it? Since I'm a beginner what are key things I should be practicing and what goals should I set for myself? I'm looking actually improve with time rather than just be stuck at a rudimentary skill level.

tl;dr new to djing
How do I build up a music collection?
Where do I start as far as mixes?
What do I practice?