You can add any type of digital object to your Omeka site (images, videos, audio, pdfs, etc). These can be born-digital objects or digital surrogates for physical objects (e.g. a photo of a sculpture).
When you click the button to add your item, make sure you check the "Public" checkbox - otherwise, visitors to your site won't be able to see your item!
Once you have a number of items uploaded to your site, you can begin to organize these items into collections. Adding a collection is much like adding a new item - you'll use the same Dublin Core metadata fields for each collection. Once you have collections, you can go back to your items screen and add each item to the appropriate collection.
Just like with individual items, make sure you have marked your collection(s) as "Public," or users won't be able to find them! You can also designate collections as "Featured," which will make the collection display on your site's home page. If you have more than one featured collection, you'll get a random one each time the home page is loaded.