"How do I know if my camera is a full frame DSLR? I have a Canon Rebel XSi. I am wanting to get a wide angle lens for shooting real estate but I don't know what to get."
A quick way to know which Canon cameras are full frame and which aren't is to look at the Dpreview.com page for Canon cameras (all the Canons made) and look in the text just above the green price. Full frame cameras say "full frame". The Rebel XSi is not full frame. It's called an APS-C or cropped sensor DSLR.
We have talked about the best choices for real estate photography lenses on Canon cropped sensors many times over the years. Probably the best choices are the following:
Most of the reviews conclude that #1 above is the best choice for the money ($279 USD). To see a list of all of the possibilities see the PFRE lens page in the section titled "For Canon 1.6x DSLRs".