Trapping an opossum can prove to be a rather long and difficult process. They are naturally cautious and will shy away from new things, like a big steel cage. Your best bet is to purchase a large live trap, which will humanely be able to trap and remove the opossum. While there are several poison's and deterrents on the market, they are not proven. Even if you are able to kill the opossum, it may die in your attic, leaving you with a terrible odor.
Always check your trap at least once, preferably more times, a day; which will keep the opossum from passing away from exhaustion or dehydration. Allowing an opossum to die in this manner is inhumane, and also the smell can attract other animals into your yard.
The best place to set your trap is in an area where the most opossum activity has been noticed. Often near gardes, trash cans, or outside of their main entry point. You can subtly hide the trap behind a tree or in shrubbery as well. It is often best to place traps alongside the fenceline in the path that the opossum takes when out scavenging for food.