This video will teach you how you can draw yourself using SketchUp so that you can include yourself as a size reference for your designs in SketchUp.
https://www.sketchup.com/education/sketchup-for-schools/curriculum
SketchUp for School has a series of videos and interactive tutorials on using the web-based version of SketchUp. If your school has allowed access to SketchUp for Schools, you can use the interactive tutorials. If not, you can still access the videos.
Basic Lessons on Using SketchUp
http://tcdstech.weebly.com/sketchup.html
Some basic lessons and assignments on using SketchUp. Mostly relevant to the Jr High level.