Once you've created your account, log in at Omeka.net and click Add a Site.
Fill in a subdomain name (this will be part of your site's URL as in, myexhibit.omeka.net) and a title. You can also enter a description of the site's content, or add it later.
When you log into Omeka to work on your site, you'll see your Dashboard.
View Site shows you the site as it will appear to the public.
Manage Site allows you to add items and collections and change settings.