This is a new style of site, taking some existing ideas and putting them together in an unusual way. If you've been here before and have already read our aims and ideals page, then great - go to it.
If you're new here, please take a moment to read the aims and ideals page, and to read about the amazing thing that makes this site work: Automatic Linking.
This site is rather unusual, and it will benefit from you understanding its purpose, why it's different, and those pages explain most of it. You will also want to read the Advice for New Users.
And now - go forth and contribute! The links at the top right of the page, duplicated and augmented at the bottom of every page, give you the most common things to try. You might want to experiment in the sandbox, or look at the test page to see the various features. You might want to read about maths mode, or simply see the recent changes.
Go, experiment, try things, and if you screw up, let me know. Backups are kept, changes are easy to revert, and mistakes are part of learning.