An ex-friend is plagiarizing me a great deal with his webcomic. I need to put my stuff online so that it will be "published" and I'll have grounds to stop him. I know of a couple of writing websites that will allow me to put my stuff online. Would that do the trick?
Not really.
I can publish lots of stuff on the internet and mark it as copyrighted, but the question is 'when exactly was it published on the internet?'.
If it is a big site and they have their own servers, then there may be a fallback to the server logs to prove when it was published, provided they would even consider helping you if it came to that.
Aside from copyrighting your work properly, which could add up in cost, you could use the old standby of letting the post office prove the date of your work by mailing it to yourself. A notary public my even sign and date your work for a small fee.
If you want to publish it online, I would recommend that you register your own domain and either set up your own website, or just set up a blog to which you can publish your work. This way it is published and you can provide a general copyright notice of all work on your site like Andrew does for this one. Since you additionally own the domain, it just looks more legit than publishing it on someone else's site where anyone can publish something, then claiming it's really your's.