I also am "playing" around with optos. The 4N25 is a good basic one, but there are some out there that are faster! You just have to get the data sheets and figure out which is best for you.
Now, to protect your PC, you will want to have separate power supplies for each side. NOTHING will be common between them. If you use Gecko drives or equivalent, the step direction connections have their own optio isolation so you only need to buffer those connections. Any other I/O should be isolated.
The input to the isolator is simply a led so you need to treat it as such. I usually set things up such that my control "sinks" the load on the cathode end of the led. So the anode end is tied to the appropriate voltage and current limit resistor. The output of many of these isolators is simply a transistor. So depending on you you want to treat it, you will connect the collector the the appropriate supply rail and the emitter to the other with the necessary resistance.
PIMO has some excellent example of how to do this.