Under US Copyright law, you cannot sue if someone stole your idea. Copyright law does not protect ideas. It only protects the expression of that idea. So the first thing you need to do is to fix your work in a tangible medium of expression (e.g., saving your work in Microsoft Word).
However, just because you write something down doesn't automatically mean it'll be copyrightable. Next, the expression has to be original (i.e., not copied) and contain a modicum of creativity (a scintilla of creativity). You should also be aware of a doctrine in copyright law called the idea/expression dichotomy. Part of that concept concerns itself with scenes a faire, which states that certain stock elements related to certain genres are not protected by copyright law (e.g., forbidden love in a romance novel). So if you were to write down the idea "Character A is forbidden by her parents to date Character B" and save it on Microsoft Word, that likely won't get you very far in a copyright lawsuit. Yes, the idea is expressed, but it contains a stock element that is common in many romance novels (forbidden love). Additionally, it may not be original (there's an argument to be made that that one sentence does not provide a scintilla of creativity needed for originality under copyright law).
Now if you were to flesh out that romance idea by constructing character profiles, outlining a plot, and creating your fictional world as detailed as possible short of actually writing your novel, you may have a potential claim. However, you still have to prove substantial similarity (but that's a blog for another idea).
So the moral of the story is if you have an idea, that's all it is under copyright law: an idea. For there to be any potential copyright law claim, get writing.
If you need any legal assistance regarding your book, like registering your copyright, negotiating your book contract, or reviewing your manuscript for legal issues, contact me here.