You can find rules and information about IDPA at their website
. Many clubs will let you compete in at least one match without joining the association.
One range I go to hosts what they call Action Pistol matches. They are conducted with USPSA rules and scoring, but they aren't sanctioned. Check out ranges in your area to see what they have to offer. If you want to get serious about competing, though, join whichever association suits you and go to sanctioned matches. That way you'll have the opportunity to qualify for national matches.
The same range that has Action Pistol competition has orientation classes. They go over the rules, show you how the targets are scored, and they set up a simple stage and let everyone shoot it for time to get a feel for a match. Maybe there's something like that near you.