VERY Open ended question.
What camera do you use at the moment?
If you already have for example a Pentax SLR then it's worth sticking with it as the lenses are compatible.
Also what's your budget? For a beginner any of entry level camera will be fine. I would however personally go Canon or Pentax because at entry level they're probabyl the best options. Nikon I'd discount due to their entry level gear not having an in-body Autofocus motor and thus not working with all their lenses.
I'd myself go for a K200D pentax if you can stretch the budget to it but the Pentax k-m ( k2000 in the US ) is a good camera and so are the Canon's although your likely to pay a little more for the canon.