Where To Start Creating Mods?

Hi, im looking to start creating mods, specifically for client side. Youtube seems to have tutorials for things like entity, block and item creation however I cant find anything for client side.

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I want to start by create simple on screen mods like a diamond counter, to track how many diamonds I have gotten in a session.

So for something simple like that, id need to check the inventory and display it. However I am not sure how to go about even this simple task. Any help would be appreciated.

If anyone would like to give advice, another mod I plan on creating is a chat macro mod, so when I press a keybind, I want it to type a command in chat. I know theres mods that do this, I just want to practice and learn, however I have no clue where to start for any of this.