You can use the full version which is downloaded at school or you can use SketchUp for School online education version for free.
SketchUp Beginners 1
SketchUp Beginners 2
SketchUp Beginners 3
Make a Sphere easily
You will need SketchUp Pro to use the Layout plugin. It is really helpful because it add notes to your plans.
Quickly scan these videos. Let me know which you think are best / worst.
SketchUp Layers
Copies & Arrays
SketchUp for Beginners 4
Make a Vase with Follow Me Tool
Make an iPhone
There are lots of different 3D Programs you can use. Some are simple, some are complex. Some need to be installed on your computer, others are online, try out 3-4 and find out which you prefer: