Android is an 'Operating System'. It is installed on smartphones and tablets by many manufacturers, eg Samsung, Asus, Lenovo, HTC, Medion, Huawei, and many many others. A problem is that each manufacturer customises the Android system to best suit their devices. This makes it difficult to create a generic tutorial. Plus, like the iOS system, the Android system gets updated regularly.
Android Basics Tutorial at GCFLearnFree is very good. Just do the tutorials in order
Complete Beginners at GadgetHacks website. They also have more advanced tutorials
Absolute Beginners Series has some online training and links to other free sources (NOT AVAILABLE as at 6/2019)
Be Connected, then click Topic Library, The absolute basics, What is a tablet.
How To Geek has a short beginners guide to getting started with an Android device
Some YouTube Videos that give clear tutorials without annoying voices and music.
- Samsung Galaxy phone Beginners Guide - relates to most Android Phones