I need advice on the new Canon Rebel I'm buying.?

So I'm going to start getting into photography because I've been interested in it for a long time but I haven't really had enough money to buy a Digital SRL. My grandma said she'd buy me my first camera for my birthday but it has to be around $500, so I'm thinking of getting the Canon Rebel XSi. I'm buying it on amazon so it's a bit cheaper. The price of the camera along with the 18-55mm lens that comes with it came up to almost $500. But I was thinking I'd get just the body and buy a separate 75-300mm lens. Would that make the picture quality better? And will it fit onto my camera? Lol. I don't know how this stuff works. Help! xD