If you do a Google search for something like 'writing forums' or 'poetry forums' then you'll find literally thousands. I've used Critique Circle, Writers Dock and Chapter 79 in the past and they're OK. It's very much a matter of having a good look around a few and seeing which one suits you. They're mostly free for a basic membership, so there's no commitment.
Don't worry too much about copyright theft. It actually happens very rarely, especially at the amateur/unpublished level. As soon as you've written your work, you become the copyright holder, and you retain that copyright unless you sign it over to someone else. It's therefore a good idea to read any website's terms and conditions before posting, as there are some who will ask for certain rights over your work. Putting it up to an online critique forum can actually work in your favour, because then you have digital date verification on it in the event you ever need it for a court case.
On the other hand, many publishers and competition organisers will consider your work published if it appears online, and this could put them off because they'd rather not get into any legal wrangles on a 'first-timer's' behalf.