I recently bought a Canon EOS 300/Rebel 2000 at a thrift store for $20.00. It came with an EF 28-90mm kit lens. I bought it because I didn't have a 35mm SLR with auto-focus and a built in flash. I think I got a good deal

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To the OP, just a few things to consider:
1) The 28mm-90mm lens I got is quite good for a kit lens, and would give you a bit more usability - especially on the wide end. In my region, they can be had for very little money used;
2) Canon sells two types of lenses. The EF lenses that will fit your camera will also fit on most of the Canon Digital SLRs, although small sensors result in a "crop factor" for all but the top range full format bodies. The Canon EF-S lenses will not fit on your camera, because they are designed for a slightly modified mount, and to cover the smaller sensors on most of the Canon DSLRs;
3) Older, non-auto-focus Canon bodies and lenses used an FD mount, which is incompatible with your camera;
4) Your camera can be used with other non-Canon mount lenses, via adapters designed to put those non-Canon mount lenses on them. That's good for someone like me, who has a reasonably diverse set of good non-Canon lenses, but I lose a lot of the functionality, such as auto-focus; and
5) There are a number of 3rd party manufacturers (Sigma, Tamron, etc.) who make lenses that will fit your camera, as well as Canon DSLRs. Some of those lenses are excellent, but it can be really confusing when you are trying to determine whether a lens can be used on a film body, or is restricted to use on the smaller sensor DSLRs.
I too recommend that you stay with what you have (or maybe a 28-90 instead of the 35-70) until you have a better idea what you like.
Welcome, and have fun!