At this stage Han, I'd suggest not getting a battery grip or flash or anything like that. It's not necessary. The built in flash will be fine for the beginning.
Upon the assumption that you get a Kit lens (18-55mm) with the body, (A safe assumption really,) the next thing you will need is a memory card. Now as it's a Canon, the chances are its a Compact Flash card you will need. If so, get this one...
http://www.play.com/Electronics/Ele...isk-Extreme-III-CompactFlash-2GB/Product.html
Play are really good. Quick delivery, reliable, and no worries about being sent a fake card.
Following this, The next essential would be a camera bag. Of course it would be silly to get a bag that can only carry the camera with just the kit lens on, because when/if you get more kit, you want to be able to take that places as well
. Something like (doesn't have to be exactly) this:
http://www.lowepro.com/Products/Shoulder_Bags/classic/Nova_3_AW.aspx
As for where to get camera bags... Either on here or on ebay is generally a good bet.
These are the essentials really. Anything after this is up to you. for instance.
If you find that a spare battery is a good idea, then its probably best to go for a battery grip. As someone has already said, doubles the battery life as the camera now holds and runs off two batteries, and the handling and functionality of the camera is much improved.
If you find you like shooting at long distances outdoors, then a telephoto lens such as the Sigma or canon 70-300mm might be a good idea, and can alsobe gotten fairly cheaply either on here or on ebay.
If you find you like shooting indoors for fassion or portraits, then a wide aperture prime, such as the nifty fifty (50mm f1.8, or if money is not so much of an issue, the f1.4 version) might be a good idea, as well as a flash gun.
Or if you are more interested in close up shots, a macro lens such as the sigma 105mm or the tamron 90mm might be a good idea
The list goes on! As said though, these are all optional. But the main things to go for, as said, are the memory card and camera bag.
HTH!