You know what you want to shoot. That's the best starting point. Don't downgrade just yet. Your XSi is a great camera and will give you an edge when shooting indoors. The ability of the camera to produce better images through it's technology is why you have it. Focal length (the ##mm) is easy to fix with an SLR. The size of the hole in the lens is called an aperture. Smaller this number more light it allows in, which allows you to freeze motion such as at a soccer game with kids running around kicking balls. Or stopping the motion mid dance to capture the smile. The combination of better images at higher ISO levels, the ability to change your lens to meet your needs and the reward you get with the quality and number of "keeper" images is why you have the camera. It's not hard to learn, and the community here will help you though. Many of your questions are answered in these forums, or through links that people will offer. If you have the time, and energy and want to learn more, stick around, ask questions, and read and absorb the knowledge that this community can give you. It might be hard to start, but everyone's had hard things to learn, and the teachers here are pretty knowledgeable.
Cheers,
