If I put a copyright sign next to my name on something I written online, will my work be protected?

As soon as you transfer it from your mind to some form of media (paper, video, audio, computer, etc.), it is protected by copyright. But, in order to enforce that protection in court, you need to register it. That can all be done online, and the US Copyright Office (assuming you are in the US - link below) has all the information you need for that. You also need to be careful of WHERE you post it. Even though you own the copyright to it, the TOS of a website could automatically give them a license to use posted content any way they wish, and might even give them ownership of the item. So be sure to carefully read the TOS . For example, just by by posting it on Facebook, you give Facebook a non-exclusive, royalty-free license to use that content as they wish. You already agree to let them do it by creating your Facebook account and posting your poems on their site. It doesn't matter if you have your copyright registered or not - it's in their TOS, which is a binding contract between you and Facebook (or what ever site you may use). Excerpt from Facecbooks TOS: "For content that is covered by intellectual property rights, like photos and videos (IP content), you specifically give us the following permission, subject to your privacy and application settings: you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook (IP License). This IP License ends when you delete your IP content or your account unless your content has been shared with others, and they have not deleted it."

The real truth is that nothing will protect you but people are extremely unlikely to copy poetry. People usually only copy something they can make money out of like romance novels. Your writing belongs to you automatically but having it belong to you legally doesn't protect you much because the law doesn't really care and stealing people's work isn't legally a crime (which I think is wrong) it's not a crime against the state you can only spend your own money to sue people if they steal work. Anyway it's actually pretty ridiculous to be worrying about anyone stealing poems. But putting the copyright symbol might possibly stop a stupid person so you may as well do it if you want to. The awful thing is that the "law" will never "get them in trouble" as I say stealing people's intellectual property is apparently not a crime against the state for some dumb reason so if someone steals your work you have to spend your own money to hire a lawyer and fight them in court which is really unfair and awful but that is how it is. Only if the judge agrees with you will they get in any "trouble" and that trouble would only amount to having to pay you for damages.

Like Mutt and Yeti say, your copyright ownership is free and automatic, and you can remind people of your ownership by using the copyright notice. However, posting your works online is an AUTOMATIC license for anyone to download and view a copy, if not also save or print one for later viewing in private. It is NOT a license for others to further distribute your works, and you can ask them to stop doing that. You can only sue in US federal courts once you have registered your copyright ownership, often by using the online form an $35 fee, which can be used to cover any number of works under one 'title', provided you are the author and they were created within the past year... Copyright does not protect any idea, concept or principle. Once you have put your ideas out there, anyone can freely copy them and there is nothing you could legally do to stop them from using them any way they want, including claiming they were the original owners. Plagiarism is not illegal. Copyright infringement is illegal.

Your work is technically protected by copyright as soon as it's in detectable form, like written down. Putting a symbol on it is now just protection against a defense of innocent infringement -- i.e., somebody doesn't get to say oh, I thought it was okay to copy it. If somebody steals your work, "the law" won't get them into trouble. You'll still have to hunt them down and sue them yourself. It probably won't be worth it to you unless they're making thousands of dollars off your work. If you put something online, for most intents and purposes, you're saying it's more worth it to you to get attention, or to get revenue from ads. Anything decent is going to get copied these days, and you have no idea where it goes once it's on the web. Put the symbol on it if you want, but you don't have to, and no symbol or anything is going to stop people copying stuff. It's still on you to sue people if there are problems.

No, but your copy right will remain intact. People will steal it, if they can make money from it.

No you need to file it with the government for copy right/trademark protection. A copy right or patent lawyer can help you file the necessary paperwork.