With the kit lens, keep it at 18mm (so you can use the widest possible aperture) and "zoom" with your feet.
Set your White Balance as low as possible, or better yet, shoot RAW and lower WB in post. (Candlelight is <2k)
Set your ISO as high as you're comfortable with. (On the 550D, I might go with 3200.)
If you're going to be shooting handheld candids, set your shutter speed to 1/60 (or faster). If you're going to be shooting static/posed shots from a tripod, you can try slowing that down to 1/30 or maybe even 1/15, but slower than that will probably result in some motion blur.
Depending on the number of candles, the pics will probably be noisy and under exposed, but that's about the best you can do without buying a faster lens. (e.g. 50mm f1.8 II)
HTH - Mark
This is good advice.
If you're getting high enough shutter speed, you could lower the maximum ISO - 1600 (or even 800 if you can get there) should be much better than 3200 on the 550D (it is on my 600D).
Assuming that there will not be any other source of light, then you are likely to only have the candles and the immediate surrounds (top of cake, close faces) lit up enough to expose - everything else will be dark - so expect this when you see your shots. This may also fool the camera meter, so don't rely on it - have a look at the images and see if you need to adjust.
Remember that at wide aperture, you will have a relatively narrow depth of field. Decide what you want to focus on before setting up your shot - i.e. do you want the face of the birthday girl (e.g.) in focus, or do you want an arty shot of the cake/candles with the people OOF.