How to do my very simple first stuff ?

Hi, I’m completely new into the drupal 8 world, and I need some advices to let me start on the code.
What I need to do first is very simple :

When a user is logged, I would like to have : a left bar in which there’s a simple text input, and when the users clicks on the “add” button at the bottom of the input, it adds the new entry into the left bar list, and so on.

Is it do-able without any module ? Which parts can I start having a look ?

THanks a lot !!!

Drupal version: 


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