The most important thing you need to compete in IPSC is a reliable handgun in 9mm or higher caliber. If you already have a gun that fits this description, great! Make sure you have a holster that covers the trigger guard, at least two magazine holders for your belt, and enough magazines to hold at least 40 rounds.
If you do not have a gun that fits the above description, do NOT go out and buy one! Come to a match or two and see what other people are shooting and then decide what you want to shoot. People shoot everything from 9mm SIGS, to 40s&w wide body 1911 variations, to Glocks, to 38super "race guns" and everything in between.
Before you compete in a match you should be intimately familiar with your handgun: how to load, unload, holster safety, draw from your holster safely, reload, clearing a jam, etc. You don't have to be fast, just safe. The most important aspect of the sport is safety.
Other things to take into consideration when coming to a match
It can get hot out there. Bring sunscreen. Bring a hat.
Bring Water, bring more water.
We tend to have stages where running and going prone are common. Have good sturdy shoes and clothes that can get dirty.
Eye protection are ears protection are always required at USPSA matches whether you are shooting or not.
At Old Bridge our matches generally last till late in the afternoon. There is no food available on the range so if you are inclined, bring lunch.
All of the clubs in this section charge $20 for a USPSA member, $25 for a non-member. Most of the matches are scheduled to start shooting at 10:00am. For the first match you plan to shoot, you need to show up BEFORE 9:30 for a safety check. When you get to the match, ask to see the Match Director and he will have a range officer give you a safety check. (of course, it is always nice to inform the Match Director before hand that a new shooter is coming)
Friendship and Camaraderie:
A large part of the fun on USPSA/IPSC Action shooting is the camaraderie that you will experience. While some people show up at a match in time to shoot and are gone shortly after the last shot is fired, a dedicated group of volunteers shows up early and stays late to build the match and then to tear it down and put it away. This group has open membership. Dues are paid in sweat. The rewards though are the highest. Don't be afraid to show up and pitch in.
Join USPSA at a match:
If interested, you can join USPSA online or you can join at a match by paying the one-year membership fee of $40 and filling out an application. You then get to shoot that match for free!
The USPSA web page has several "New Shooter" articles that may interest you.