You mean 18-55, right? That's the standard kit lens, and it's a good one. Can't comment on the Sigma lens, although it reviews well on Amazon and elsewhere.
Do yourself a favour - get a good fast prime, ideally the 40mm pancake. I've got the 50mm f/1.8 and whilst it's a remarkably good lens for the money (not much, about seventy nicker) on a crop body it's a bit long for candids in a social setting. OTOH, it's a great lens for candids in a social setting because you don't have to get too close to people, thus preventing some of the "oh a camera i must gurn for it" stuff.
Also, get a tripod - Ken Rockwell says you don't need one, and most of the time he's right, but the high ISO performance of the 550D isn't particularly stellar, so a tripod it is. Don't spend more than $50-60 at this point. I've got a cheap and cheerful Manfrotto which is light, stable and has a nifty universal adjustment thingamabob. It's not very strong, but it cost buttons and it folds up very small.
http://www.amazon.co...ack/dp/B004MKNJ84
Anyway, gear's boring. Take pictures!