If you are lucky the homepage you created on your computer is also listed. If it is, all you have to do is put a check mark in front of index.html and delete it ("Delete" button etc.), then put a check mark in front of your home page filename and rename it ("Rename" button} to index.html.
If your home page file is not there you must either create it at GeoCities with one of the editors in file manager or upload it by using either EZ Upload or an FTP client like WS_FTP. You'll find instructions either here for AOL or at answerpoint for a general purpose client.