If you took advantage of Amazon's deal on the Canon Xsi like I did, there are a few things you'll need. The deal, which is still alive, gets you the Canon XSI with EF-S 18-55mm f/3.5-5.6 lens and the Canon EF-S 55-250mm f/4.0-5.6 IS Telephoto lens, all for $614. (Here's the link to my post on how to get that price).
So you've got the camera body and two lenses. First, you need a good bag, one that will hold your lenses and room for additional equipment. I tried buying one online, but I found that I needed to physically inspect some bags prior to purchasing. So I went to one of the oldest camera stores in the Sacramento area, which has been around since 1946. It's called Pardee's Cameras, and they had dozens of bags. The one I liked the best was from Canon, the 100DG Bag. While I could have simply gone home and bought it cheaper on the Internet, the staff was so helpful that I felt obliged to buy it from them. This bag is big, with multiple compartments. You can even fit a netbook in it. While I like it, my wife thinks it's too big. You can buy it here for $69.95:
While the built in flash on the XSi works, I went ahead and bought the Speedlite 430EX II for $250 from Amazon:
You'll also need a lens hood and filter for each lens.
EF-S 18-55mm f/3.5-5.6 lens
The hood for this camera is the Canon EW-60C. It's $19.37 on Amazon.
Canon EF-S 55-250mm f/4.0-5.6 IS Telephoto lens
For the filter, I recommend the Hoya 58mm UV (Ultra Violet) Multi Coated Glass Filter. It's $22.36 on Amazon.
The hood for this lens is the Canon ET60. Although the description says it's for the EF75-300mm, it fits the 55-250 as well. It's $23.99.
I haven't bought it yet, but my next purchase will be the Canon BG-E5 Battery Grip. It's $111.95 on Amazon.