Supply is pretty basic and the tutorial did a fair job or covering it, as there's not much to it. (on a basic level, at least) Have you replayed the boot camp mission that covers it to try reading the tutorial pop-up explanation for supply again in case you missed something?
Otherwise, I'd go ahead and ask any questions you have here and see if we can help you.
Basically when you press space you'll see units with black bubbles, these are total supply they offer, the green number on units such as infantry when pressing spacee bar is the supply they take, the number in green in the corner of the black bubbles is the supply remaining that the unit has left after any units it's supplying.
For an easy example, if you have a small island and on this small island you have a supply ship worth 20 supply (as you can see when you press the space bar by the big 20 in the black bubble) and two infantry units that take 5 supply each (as you can see from the number on the infantry units when hitting space) When pressing space and looking at our small island as a whole you'll see...
a 20 in the supply ship black bubble and in the corner of that bubble you'll see a green 10 because that supply ship originally had 20 supplies and 10 of those supplies are being taken up by our two infantry units that cost 5 supply each, which leaves ten for the ship.
So if you want to place a new unit now you'll need to place one that costs 10 or less or place a unit that provides supply such as a ship OR capture a supply point and wait a few turns for it to capture all the way and provide supply.
You may or may not have the unit allowance to place more units at all, in which capturing certain points on the map would allow you to place more.
I'm not very good at explaining things but I promise it's extremely basic when it comes to supply and you should have no problem eventually understanding it.
Also, putting into YouTube "Order of Battle Supply Tutorial" provides an in-depth tutorial as the first result which will also help you.
Forgive me if my explanation covered things you already knew such as the basics, you didn't really provide any information in your post regarding WHAT you were struggling with in regards to supply so I thought maybe just a quick explanations of the basics would help.
If you have more questions ask here with specifics on what you're struggling with or watch this video:
https://youtu.be/MtUJwK04drc?si=IQPA15g3P8TfthVa
Have fun!
Please let us know what you're struggling with or bt