I'm in a similar situation: getting into IDPA. kraigwy has some good advice, especially if you're only getting into competition for a project and don't see it developing into something permanent.
If you're looking to combine your purchase for competition with a SD piece or just a range gun, then I would advise that you visit a local gun store and check out whatever you might be interested in. Actually handle some different models to see what fits your hand and points naturally for you. (Be it a revolver or an auto) Whatever is comfortable to you is key here. Competition shooting induces stress, so the more comfortable you are with your gun, the better.
As for "tactical" guns without external safety features, Sigs are great. Expensive, but great. Glocks are a cheaper, similar option (have internal safeties that disengage as you pull the trigger). Both brands have models in .9mm, .357 sig, .40 S&W and .45 ACP.
As for caliber...if you're not too familiar with shooting, or if your competition specifies which caliber you can use, then I'd recommend something light like a .9mm. Competitions like IDPA require a .9mm or above for pistols. Make sure to check to see what the caliber restrictions are for whatever competition you're doing. Best of luck and be safe!
Last edited by Ambishot; September 1, 2011 at 10:16 AM.