How to create a text file (or any file) in J2ME and how to write to it, and how to open an existing file, seek to position (such as th end) and write more to it, is documented in the SDK where the FileConnection API is documented. I propose you read and study it.
Finding the available root file systems, etc., is also documented, for example, in the article I just posted previously. I suggest you study it, too, as well as the other FileConnection articles and examples in the wiki. It is really quite a straight-forward API. Just remember, that you're not allowed to create files anywhere you like (and where you are allowed varies by the target device/platform).