I assume you are talking about acrylic sheets and not rod? If it was a rod just use a saw of some sort :-)
If it is a sheet, you'll want an acrylic cutter from a hardware store to make your life easier. It looks like a regular utility knife/blade but is hooked at the end of it.
Lay your acrylic sheet down and score the acrylic where you want to cut it. Score it a few times on either side. Then snap the acrylic where you scored it.
