The Omeka.net documentation has more information than is contained on this guide. Check it out if you're feeling stuck! You can also contact the library to get more help.
Dublin Core is a general metadata schema that is meant to work for many different kinds of objects. However, with its limited fields, it can be hard to thoroughly capture the properties of your items. Item Type Metadata can be used to supplement the metadata you created using Dublin Core. Omeka has a list of Item Types that are built into the platform.
If you have an Item Type that's not in Omeka's list, you can create a new one or edit an existing Item type.
Use the arrows at the bottom of the page to learn more about the next step of creating an item: adding files to represent your item.