How do I delete jailbroken apps?

cydia->manage->packages, click on the app you want to un-install and the option will be on the top of the page.

Installer->categories->installed packages, same as cydia.

If you're trying to uninstall Installer, you can SSH into the phone (if you have it installed, otherwise do a search), and remove Installer.app from the Applications directory.

As for uninstalling installed packages, just be careful that you're not removing anything that has a dependency from another package. I'm not overly familiar with how well it checks dependencies, but if something you do use suddenly stops working, something else you uninstalled is likely the culprit.