By and large, anything you post publicly available online is copyrighted to you even if you did not necessarily file out all the paperwork needed to be so. Having a good track record and sourcing of your original work will do good, but ultimately there's not much that can be done about it being "stolen" in so far as someone taking a copy of it, posting it elsewhere, and claiming credit.

If you want to vigorously hold onto your ideas then you just don't want to talk about them to anyone but people you trust. You can't copyright an idea and there's nothing stopping someone from being successful with an idea you posited. The Inheritance series of novels by Paolini is consistently criticized as a fantasy rewrite of Star Wars but there's not much LucasArts et al. can do with that.

If you're interested in building a portfolio of work, then sharing your writing is a way to advance that goal and therefore sharing is in your interests. If you just want to share some writing because you want others to read it, and have no particular considerations about monetization, I would avoid obsessing about the plagiarism/theft thing. Not that it isn't important, but it's not an immediate consideration.

Do not make publicly available anything you hope to monetize.

I think in early stages it would be better to just concentrate on getting better. On the internet there's not any way to stop your writing from potentially being redistributed, but likely no one will want to steal it either.