If you have a VERY bad back, you should really get professional advice before exercising. See a chiropractor/osteopath and get it fixed.
Things to avoid: Heavy squats, deadlifts, poor form of any sort.
Work on your "core" muscles (see other threads about this one) - these are the little muscles you can't see which stabilize the spine. They're quick to get weak when we have back pain.
Just take it easy and be kind to yourself - sharp pain is bad, aching is ok (unless it never goes away). The fact that you're exercising at all is worth a lot.
