Underground, you can use a Fernco coupling to attach the new ABS to the cast iron that you'll have to cut. There are several ways to do that: saw, grinder, snap cutter.
You want the trap riser directly under the new shower drain. It depends on what kind of a shower you are going to do where that drain goes. Make sure you get the trap arm aligned with the rest of the drain and maintain the 1/4" per foot slope of the horizontal section.
Once you get the plumbing straight, if you are going to tile it, check out www.johnbridge.com for help in constructing the shower properly. The specific type of drain you use will depend on the method you use to build the shower.