If your question is regarding modeling tools, there are quite a few options, with Rational
Rose and Together being some of the better known ones. Together actually has a free version called Together Whiteboard, which is limited to class diagrams - this might be all you need to get started. You can download this from "http://www.togethersoft.com".
I don't think that you necessarily need a tool with reverse/forward (i.e. roundtrip) engineering, although I think that Together supports this. Initially, you won't have any classes to reverse engineer into a class diagram anyway. It might be helpful if the tool can generate the code framework for you, and later on even better if it also supports roundtrip engineering, especially if you do a lot of refactoring.
-Mirko