I'd say one helpful thing is to understand a few fundamentals in Construct 3, and I'm not talking about the typical "add the platform behaviour and you're done" kind of basics.
My suggestion, is to get VERY familiar with some common objects, such as:
9-patch (useful for HUD elements, things like square windows, or when a highlight box appears if you wanted to click and drag to highlight units on an RTS game)
And familiarise yourself with concepts in the event sheet view, such as:
For Each loops
Without writing about literally everything in Construct, I'll go on about Arrays and Dictionaries for an inventory system.
Before reading the below, if you don't know what arrays and dictionarys are, I'd suggest reading the documentation on Construct 3's website about these two objects first.
So, Arrays are an amazing way to setup some data that you can read or write to - it would be good for storing the information for an inventory system, as you can have your array grown and shrink depending on what items are picked up ingame, or if an item is used. Arrays can be setup to store information about each inventory slot such as "quantity" and "damage", and you can then design your events so that when you "use" an item, it doesn't just delete the item from the array, but it would first check the quantity, subtract from it, and if the quantity was only "1", THEN it can delete the entry from the array.
The above is a random thought idea for designing an inventory system - you could design an inventory system with a Dictionary object instead, but you may find there are pros and cons to whatever choice you make. Dictionary objects are kinda similar to arrays, but they are maybe "easier" to use, but come at a possible performance cost if you add thousands of items, or there may be a "management" cost if you wanted to be able to store lots of details about each item slot (an array you could simply "delete" the item slot in 1 event - but with a dictionary, you would need to loop through every key and find anything related to the item slot, e.g. If you named all your item slot dictionary keys like "slot0_Name", "slot0_Quantity").
As you're new, I'd say try and familiarise yourself first with the concepts I listed at the start of this post, don't worry so much about performance right away, but worry about getting things to WORK correctly. Once you're more aware and feeling more confident, then you can start thinking more about performance, which is another entire thought process, this comes from lots of reading and experimenting, for example, you'll come to find that "collisions" are quite expensive for the CPU, so you will want to design your events to not check for collisions every moment, but instead try and hide the collision checks away in sub-events and make sure other easier checks are done first, or maybe disabling collisions for objects that don't need checking at the time, etc.
And honestly, in general, keep reading the documentation often, Scirra have done a great job at writing everything down about literally everything within construct 3, like last year I was casually reading about the "Touch" object, and I found out randomly by reading on the documentation that there is support for "pen sensitivity" - how cool is that!? It works with my Note 9 S-Pen. Nobody ever really talks about that on the forums, but it's really useful to know what capabilities Construct 3 has. Scirra have also written some tips and useful info on pretty much every page on the documentation, sometimes they write warnings like "try not to use this object for blah blah, instead, use blah blah" it's super useful stuff!
ALSO I'd suggest googling small questions you have, followed by "Construct 3". Such as "is it better to use array or dictionary construct 3". By doing a search like this, you might find older posts with people discussing these concepts indepth, and you might find a useful piece of info or two! Be aware of the date of the forum post though, as it might be a really old post and may be irrelevant these days.
You could find tutorials that provide the c3p file on the specific questions that you asked, such as "how do I make a inventory system?", then you could open these up and try to read the events (hopefully there are comments in the events too!), and if unsure about an event, you could poke around the event, change a variable or disable an event or action, to see what it changes - once something breaks, you'll realise "ohh that one event was to prevent blah blah from happening" or "oh this event completely breaks the inventory, so it must be important". If you get completely stuck on what something is, you can always post on these forums and see what the community thinks!
Hope this is useful in some way!