Hello and welcome!
Google Sketchup FAQ: http://code.google.com/intl/nb/apis/sketchup/docs/faq.html
The API manual: http://code.google.com/intl/nb/apis/sketchup/docs/index.html
More links and info in the Stickies in this forum section - such as this one: http://forums.sketchucation.com/viewtopic.php?f=180&t=10142
Some other tips:
Wrap your code in a module to avoid conflicts with other scripts.
Don't modify base classes - as other scripts might rely on their original behaviour.
Beware observers - many are bugged.