I carry a Xsi (after years of using "higher end" Canons) and make no mistake, the Xsi is a very capable camera! The kit lens is pretty good too, just not at it's best in low light or wide open.
Tips? Set the camera to use only the center focus point, it will focus faster and more accurately. Practice aiming the focus point at an area with high contrast to insure quick focus.
Use RAW, or RAW + JPG if you really need processed images. RAW will capture much more detail and you will be glad you did this in years to come, even if you do not use it now. This bears repeating because all it takes is more memory cards and they are inexpensive but memories are not! Use RAW!
Use Program mode if you are not sure of yourself. No, it is not what many photographers consider "true" photography but it will deliver a well exposed photo automatically in almost any lighting condition, something that Av, Tv, and especially M will not. I almost mentioned using Auto (green rectangle) but that will not allow using RAW.
ISO 1600 is usable but not without some noise reduction software. Be careful not to underexpose at high ISO. *Always* remember to set ISO back to lower levels when done with low light shooting!
Read the manual. Read the manual, again. Even twice is not enough, there are a lot of settings with which to get ourselves in trouble.
Practice a smooth shutter release, better yet use a monopod or some camera support. Even photos taken at 1/30 can have a lot of blur without good technique.
Oh yes, have fun!
