Hello g highfield

No .. You do not require any plugins to do most 3D Modeling using SketchUp.
Plugins are tools are often created and used simply to simplify many functions that are easy enough to do with SU's native tools alone.
Some are more specific and offer functions that are difficult if not impossible to do with SU's native tools alone.

I recommend watching (and following along) ALL of the Video Tutorials available on Google SketchUp's Tutorial site (especially ALL the beginner videos). The little time spent watching and practicing (a few hours only) will be time well spent.

Then you can also find Tons of Video Tutorials on YouTube.

Happy Modeling