For a detailed guide to Omeka, try the Omeka Codex. For help in setting up an exhibit, try this guide from Miriam Posner.
Below, a quick guide to getting started with adding items in Omeka:
OMEKA: Exhibit Builder
by Anthony Bushong and David Kim 
What is Omeka? 
Omeka is a web publishing platform and a content management system (CMS), developed by the Center for History and New Media (CHNM) at George Mason University. Omeka was developed specifically for scholarly content, with particular emphasis on digital collections and exhibits. While Omeka may not be as readily customizable as other platforms designed for general use, such as WordPress, Omeka has been used by many academic and cultural institutions for its built-in features for cataloging and presenting digital collections. Developing content in Omeka is complemented by an extensive list of descriptive metadata fields that conforms to Dublin Core, a standard used by libraries, museums and archives (for more on metadata and creating a data repository, click through to the creating a repository section). This additional layer helps to establish proper source attribution, standards for description and organization of digital resources–all important aspects of scholarly work in classroom settings but often overlooked in general blogging platforms. 

Building a Repository in Omeka
1) Add Items You can add almost all popular file formats in Omeka for images, video, sound and documents. When adding an item, you will start with at your Dashboard. a. Select Add a New Item to Your Archive under the ‘Items’ heading. 
2) Descriptive Metadata When you add items in Omeka, you are required to use Dublin Core Metadata Element Set. Click here to learn about the vocabulary used in Dublin Core. a. Use this taxonomy to describe the item that you are adding. b. Make sure you group decides on standards to describe various aspects of the items: (date: by year, century, span?), (subject: Library of Congress Subject Heading?) (location: City and State, Country, region?) You don’t have the use all Dublin Core fields included with Omeka, but the selection of the fields you choose to describe should be consistent for all items. c. Next, select Item Type Metadata. In this section, you can select amongst 12 different item categories under Item Type. These metadata fields are specific to each of their respective types.
3) Tags You can use tags to help make your items easily searchable based on the classification that your group have decided are relevant not only to the item but to the general scheme of your overall project. Tags are also often referred to as folksonomies. 
4) Assign the item to a ‘collection’ The collection types should be based off of how you desire to organize your items. If you want to add a new collection, go to Collections -> Add a New Collection in the top right hand corner. These collections should reflect the different types of items and should be useful for referencing items in your exhibits. 
5) Creating Exhibits Exhibits make use of the items in the collection to create visual narratives. The Exhibit Builder plug-in offers several template options for the individual sections and pages within your exhibit. First, understand the hierarchy of the exhibits: Exhibits — Sections — Pages. Then, take a moment to sketch out the organization of the exhibit prior to creating them in Omeka. Watch this video for step-by-step process.
6) Non-Exhibit Content a. Omeka offers the Simple Pages plug-in to create pages within your Omeka site that are not associated with any specific exhibits, such as the home page and the “about” page. b. Omeka provides many instructions for various activities. * See its documentation page for a list of solutions for common problems and suggestions for embedding Google maps, YouTube videos, etc.
Available on the web at the UCLA DH Center Intro to Digital Humanities