Decide on the dimension of your pen and what it is to be made of. Try to avoid chicken wire as it does a good job of keeping chickens in but a poor job of keeping predators out. You can use it as fencing but at the very least use hardware cloth around the perimeter about 18" up (even bury it a few inches to keep digging predators at bay). HW cloth prevents raccoons from reaching through chicken wire and snagging a chicken and eating it from the other side of the wire. Depending on your situation you may also want to cover the top to keep out flying and climbing predators. Keep in mind, chicken wire is cheap - but even with HW cloth around the bottom, a determined predator (like a dog) can get through chicken wire with enough time. Good luck to you!