If you really want to understand Telegram API development from scratch. My advice would be to follow the steps here
https://core.telegram.org/mtproto/auth_key
and here
https://core.telegram.org/mtproto/samples-auth_key
Try to successfully generate an AuthKey.
This exercise will get you familiar with enough of the basics as well as help you build up routines you will need to do further work on Telegram API.
I have outlined the basics for you to get started in this SO post.
Also i think the API documentation online is not so well-written, but following the above step by step while reading the API documentation, for just AuthKey generation, would get you familiar with the language and writing style of the authors of the API
Good Luck.