Welcome to the site!
If you need help with changing fonts and such, this guide should help you with all of that... "
Posting - What are those buttons???"
As for changing your mood, you should a little area that looks like this above the thread, on the right hand side...
... just click on the mood picture beside "My Mood:" and a drop down list with all the current moods will appear. To change it just click the new one you want.
Hope that helps, and if you need help with anything else I'll try to help with that, too.