If you've never had purposely aged beer before I'd recommend one of two things to start out. The slow way would be to buy a case of something that ages well, big stouts and barleywine's being the most common. Taste fresh, and then continue pulling them out on occasion. Once a month is a good idea as you'll probably not notice anything real early, but as you get about halfway through the case you'll typically start to notice the difference. Some beers improve, some beers degrade. Many factors are out of your control. All you can do is keep them cool and away from light and hope for the best.
Option two is usually more expensive and you'll learn less. That is to find a bottle shop that cellars and sells aged beers. Finding one beer with multiple vintages, even if it's only the fresh one and last years will give you an idea of what aging might do for that beer.
In general, I find most beers to be best enjoyed fresh. I like age on some beers, but for me it's more fun to taste the beer as it ages. I don't think I'd ever age something purposely that I didn't have a few bottles of.