Most folks find the video tutorials are the easiest way to learn SketchUp.
There’s practically no end to learning resources, some* of which are built into the SketchUp user interface.
*• Instructor (Click … Window > Instructor)
http://help.sketchup.com/en/article/114972
*• Status Bar (Located at the bottom of the SketchUp drawing area)
http://help.sketchup.com/en/article/70066
• Learning SketchUp
http://help.sketchup.com/en/article/36207
• Quick Reference Card
http://help.sketchup.com/en/article/116693
• Getting Started Guide
http://help.sketchup.com/en/article/95079
• Concepts Guide
http://help.sketchup.com/en/article/115426
• SketchUp User’s Guide
http://help.sketchup.com/en/article/116174
• SketchUp Knowledge Center
http://help.sketchup.com/en
• SketchUp Training Videos (latest)
• SketchUp Training Videos (older)
• SketchUp Training Videos (SketchUp site)
http://www.sketchup.com/learn/videos/58
• Aidan Chopra Video Tutorials
•The SketchUp Sage and SUWiki
• Trimble SketchUp Help Forum
http://forums.sketchup.com/
• SketchUcation Community Forums
http://sketchucation.com/forums/