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:
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
SketchUp Layers
Copies & Arrays
SketchUp for Beginners 4
Make a Vase with Follow Me Tool
Make an iPhone