Hi there!
Rather than just say "read the forums" - because there are MANY SR20 conversions on this forum, basically you'll need to swap the engine (derp), fuel rails, injectors, loom, ecu, and exhaust/intake manifolds. You'll also need to weld in a new engine mount, I believe - if you've got no experience with this sort of thing, find someone who does.
Converting from FWD to AWD is a massive problem - you've gotta run a drive tunnel to the rear axle, and the chassis of our cars doesn't allow for it. Someone on here has done it, but they got a workshop to do it for them - cost about AU$10k. It'd probably be half as much in the US, or possibly even less. You'd have to swap over all the running gear - the suspension units in the u12 bluebirds are different, so watch that - if you're able to do this all yourself, prepare for a mind fuck. But the tunnel needs to be worked into the chassis. It's a big operation if you wanna go AWD. not just tack off the AWD running gear and bolt on.
If you want more information about the SR20 conversion, go into the Project Threads section, and read up on Kranzy's SR20 conversion - plenty of details. If you've got any specifics, just keep asking away.
Oh, and Cru1se_Missile did the AWD conversion, fwiw - check his project thread for a bit of info.