How can I be sure no one will steal my ideas?
I hate to be blunt, but it's the honest truth. The unfortunate reality is that anything you post on the Internet, anywhere at all, has at least a remote chance of being stolen or co-opted by anyone who sees it at any given time. Even if you delete your questions the moment they're answered (which is very bad practice, so please don't), I have enough rep to see deleted posts, so that won't protect you completely.
I have the same concerns about getting my ideas stolen, but I've come to accept that it's just one of the perils of using the Internet, and that there's not a whole lot I can do about it. But more importantly: I trust the community here. I choose to believe that they wouldn't do that. I'm sure they all trust each other not to do the same as well. So while this may not be the answer you want to hear, you're just going to have to trust us.
(And remember: even if someone does steal one of your ideas, they've still gotta actually write the damn thing. And that's the hard part. Chances are they won't manage it.)