You design a circuit and then simulate it with LTspice to see if it works as you designed it.
You start the design by defining the input and output signals you want (analog/digital, frequency, voltage, impedance, etc.), select the appropriate opto isolator and go from there.
Sound like a school assignment. I'd start by downloading ltspice to your PC and installing it. Then find one of the many on-line tutorials to learn how to use it. Then do what crutschow said.
by the way, in the future, the more specific and detailed your questions are, the better the chance that you'll get a useful answer. Also, if it looks like you have actually tried to understand the problem and have researched for solutions, you will have a better chance that you'll get a useful answer.