Inventory basics + bars

So I want to create an inventory by myself. Without any tutorials, becouse I feel I dont learn nothing and my interes is to learn C# and mayby java ( I consider Java a toy, becouse at game development centers such as ubisoft use c#). But I need some basics from where I need to start. So my idea is to create a inventory with a background a backpack, and you need to roatate and arrange things like in real live to get more. 1) I need to know how I can make some slots where I can put items who can be bigger by 6-1 slots 2) To put one item in hands to have a little delay who is represented by a circle who is likely puppet’s from fnaf (sorry, I dont know hoe to name that) 3) I want nearly that backpack a layout with a human sketch where you can put cloths
I dont want you to make me an inventory, I just want to show me some methonds good enought from where I can learn and things
Background I will make in photoshop and manga studio.
And a little of topic, it is possible to make a var to increase or decrease smooth
Sorry my grammar, what you see here is learned from games and youtube

up, please, I dont want premaded, I want just some hints. If you dont know all of this, just write what you know