Opinions are like...well, you know.
Here's mine.
If you're a new shooter getting into pistol competition, most of the time that means you're thinking about some type of "action" pistol - either Steel Challenge, IDPA, or USPSA. What I'm going to ask you to consider is whether this is a "I'd like to try this with a gun I want for some other reason" or a "I really think I'd like to be a competitive shooter, and I want a good gun to do it with" situation. If it's the latter, here's my suggestion.
Glock 17. No Sigmas, no M&P's, no CZ's, no XD's. Find a used Glock 17. If you don't already own a centerfire pistol, you've got no reason to go with anything else, in my opinion (and hey, I don't even like Glocks!)
The Glock 17 is a full-size 9mm, with a well-supported chamber. That doesn't mean anything to you right now, but it may in time. 9mm is the cheapest centerfire pistol ammo you're going to be able to buy for competition (until you start reloading). The Glock is second only to the 1911 in support (spare parts, aftermarket triggers, barrels, etc) - all of which are critically important to a competition shooter.
The 9mm will score lower in some of the games, but it allows you to get into the competition quickly and at the least cost. The other advantage (IMHO) is that you're learning not just that gun, but the Glock system. Want to shoot limited? You can eventually pick up a Glock 22 or 35. Build an open gun? Maybe you build a 9mm Major gun on a Glock 34. Want a concealed carry gun? A Glock 23 is just the ticket. The guns will be familiar and all work the same. The only other platforms that offer that are the 1911, CZ, and maybe the Beretta, but parts for the CZ and Beretta are MUCH harder to come by. Magazines and parts for the Glock are available everywhere, and there will be specialty dealers set up at every major gun show to support the Glock enthusiast.
Try to find a used Glock 17. Maybe it's not a police trade-in, maybe it's a private seller in your local paper. Some gun ranges have a bulletin board where folks post their guns for sale. Keep going to gun shows, and know that Cash is King - you can often get a better deal by starting with $100 bills than by pulling out a checkbook or credit card.
Again, IMHO, if you can't afford to get a used Glock, you can't really afford to get into competition shooting. If you're going to an indoor range, expect each session to run you around $40 (that's $15 for the range time, $25 for 100 rounds of ammo). Competitions (the cheap ones) will run you about $75-$90 for range fees and ammo. You need to be going to the range a at least once a week to get familiar with the gun and how you shoot it. Scrimp and save for the right gun - once you've shot a dozen competitions, you've already spent more on ammo than you did on the gun!
Now, the next thing I'm going to suggest is that you buy a clear plastic shoe box or a 5 gallon bucket. Every time you buy ammo, buy the Winchester White Box 100 round boxes from Wal-Mart. When you go to the range and shoot, try to pick up every one of your empty cases. Put 'em in a baggie, and dump them in the shoe box (or bucket) when you get home. If you really get serious about competition, you'll be thanking yourself down the road. Because IMHO the big savings in getting into competition is not in a cheaper gun, it's in getting into reloading when you're a year into competition. Save your brass, save your pennies, and plan on buying a reloader if you really like this.
Sorry to be long winded, but I've been exactly where you were. I saved up my pennies and bought the gun I really wanted to shoot USPSA with to start (a Kimber 1911). I almost bought a cheaper gun to start with. I am SO glad I didn't, as I've had my Kimber for almost 15 years and will never sell it. The cheaper guns I bought afterwards are long gone.
Buy that used Glock. I bet you can't wear it out.