When you log-in, click the "create content" button. You'll then have to create a character for testing purposes (and it WILL take up one of your character slots), and then (I believe) the create content button again.
That'll take you into the editor... and I simply dont have the time to fully explain how all of that works. There's plenty of tutorials floating around, and if you can't figure something out then don't hesitate to ask here on the boards. They'll either tell you how to do it, or tell you that it's something that can't be done (Foundry has its limitations).
How do you create missions in the community authered section?
Your best bet as well is to watch the tutorial series that are available. Go to http://starbaseugc.com/ and the site will have a great deal of tutorials and other information regarding the Foundry and its many uses. You can also get help in the chat room on specific questions if you have any. The Foundry is a great tool, and the tutorials will help you immensely in how to use the many options available to you. Good Luck and Hope to see you there soon.