You can't delete all of your questions and answers. You've granted us the irrevocable right to display your content on this website. That's the license you agreed to when you joined Stack Overflow.
You can, however, delete your account, causing your contributions to be anonymized. See here for details. Basically, modify your profile to say "delete this account", and then flag one of your posts for moderator attention.
As for why your reputation points are being decreased, I suspect that's because one (or more) of your posts were flagged as "spam" by at least 6 other members of the community. Each spam post deletion costs the owning account -100 reputation points.
If you've been posting links back to your personal blog, or mass-posting the same answer to a bunch of different questions, it's a good bet that this is what happened to you. If you stop, the rep-loss will not continue. Unfortunately, there's no way to get back those points, other than by contributing useful content to the community and getting upvotes.