Blog Security Holes?

[JYM]

What are a blog's main security holes? I'm thinking about using WordPress and not only that but I plan to make a very popular blog which should generate quite a bit of money. I'm afraid that hackers might hack it if they get word, just like some hackers did to milliondollarhomepage (which doesn't even use mysql databases...). I heard there's someting called sql injection which is a hacking technique, how safe do you think wordpress is?