Well I wrote a nice long post then it dispersed, so here is the short and sweet version.
Things you will need:
Dash Kit-Needed if not using the stock radio
Wiring Harness-Not really needed, but unless you want to cut wires...then yes
Steering Wheel Control Unit-If you want to keep your functionality you will need this. I used the Metra Axxess and besides the confusing wire colors they use it works great(also the car wiring harness will not come with these wires normally so you will need to add them or buy one that has them)
Plastic trim removal tool-Not 100% needed, but helps alot
To get the USB port to work you will need to run a cord to the center consul. It needs a male USB end. You could also cut the stock plug off and wire a USB port on there.
Here are a couple of links I found extremely helpful when I attempted this:
Subaru radio and radio wiring harness information Great wiring info and he also sells a harness with the steering wheel wires on it already.
http://www.clubwrx.net/forums/tutori...od-thread.html Has a manual for the car. Wiring and removal info is in here.
Well good luck