You start by gathering as much information as you can, this link has a ton of information
TweakHeadz Lab Electronic Musician's Hangout
I know as a 15 year old youre not going to want to hear this but the more preparation you do the quicker you'll get good results..
Also consider what part each member is going to play in this band, are you going to produce all the music yourself, are you going to want to play instruments, are you going to track and mix from someones home...
If you can divvy up responsibilities in the beginning you can focus on the roles you have top learn
By far the best software for beats/electronic related music is Ableton Live...but realise that will take up $300 of your budget so download the month long demo and see how you get on with it...youll need an audio interface, perhaps just two inputs will do if your not expecting to record a huge amount of instrument.. try out some mics, you need to try them to see if you voice matches the mic..there are mic around the $100 that will get you a good result (MXLv67g)
are you going to play soft synths? try maybe an Axiom..it has drum pads and a great keyboard, or a interface like NIs Machine has everything..at $500 its not cheap but thats hardware and software
this is all presuming you already have a PC....soft synths like Tone2's Gladiator can cover most sounds but NI also have some great packages..research research..I but mags..future music and computer music show you how the pros make tracks and have loads of freebie
keep things within a budget and spend as much as you can on nearfield monitors, KRKs are pretty popular...take your fave CD down to the music shop and play it on different sets...which has the most detail
its a mountain to begin with but get started and think of specific questions..folks will clamber to help you in here but folks do hate repetitive questions that are easy to find the answers too and over generalizations..be specific, and try to get out as much information as possible
welcome aboard