I’m going to make a LOT of assumptions based on a very vague description of a GUI health bar and inventory system.
Starting with the health bar I will assume that each bar reperesenets a percentage of health, ~14% health for each bar, for this one you pretty much just need to check if the health is between a certain point and drop a bar.
If you are talking about making it slide from one side to another, this looks like a decent tutorial.
For an inventory system, you will need to first figure out how you are going to store player information if you already have this sorted then how to integrate it. You can choose to use Roblox’s built in DataStore service or use HTTPService to call to an external PHP server where you can store to a MySQL DB, but that’s complicated and requires you to keep the server online.
There are also some open-source modules like ProfileService or Datastore2 which are actually pretty awesome tools for beginners or even the most professional users.
Whether that is your UI or not displayed you will need to structure it in studio, set it up and write the code for it to have animations, interactions etc.
Next is coding the actual storing of information and thats about it, a couple button clicks etc. Inventories are usually simple but you need to really think through how you are doing things even restarting once or twice isn’t a terrible idea as you do not want any little bugs involved in inventories.
To get started, stop posting on the dev-forum and procrastinating, you know how to do some basic GUI work so just start messing around, break it, then break it again. You may not know where to start, but neither did anyone who opened studio for the first time, let alone treading into the unknown.