Open Cydia. It's a brown app with a box icon on it. You will find it on the Home Screen after the jailbreak completes, though you will have to scroll to a Home Screen page to view it if your primary Home Screen is full.
Review the tabs at the bottom of Cydia. These tabs include the following:
Cydia - Bottom-left corner of the screen. This is the Cydia Home page.
Sources - Right of Cydia. Any repositories, which are places from which you can download tweaks, will appear here. You can add repositories by tapping Edit in the top-right corner of the screen, tapping Add in the top-left corner, typing in a repository's URL, and tapping Add Source.
Changes - Right of Sources. This page is analogous to the Updates tab in the iOS App Store. To update your iOS device's tweaks and apps, tap Upgrade in the top-right corner of the screen.
Installed - Right of Changes. You can view a comprehensive list of all apps, tweaks, and other changes. To remove a change, tap it, tap Modify in the top-right corner, and tap Remove.
Search - Bottom-right corner of the screen. Allows you to search for Cydia apps, tweaks, etc.
Tap Cydia. You'll be taken back to the Home page.
Tap Themes. It's on the top-right side of the screen. You can browse Cydia themes, which fundamentally change the way your iOS device's display appears and reacts, in this section.
The majority of the tweaks here are paid.
Continue browsing Cydia. As you navigate Cydia, you will get a feel for the kinds of themes or tweaks you prefer, and you'll be able to download them as you go.