Literally try to do everything you think of and when you are confused, hit a wall, encounter unexpected behavior, etc. look to the forums and ask specific/pointed questions. The more you experiment by simply using the editor, the more you'll learn.
I'll link the two trigger-related tutorials that I find myself referring others to most often:
For a structured tutorial on the matter, read this. Hello guys! Credit to: Wolverabid, PurplePoot, Silvenon, Paskovich I want to make a list of things that leak and ways to fix them. Dynamic Unit Groups This trigger will instantly destroy dynamically generated unit groups. Trigger...
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Contents Introduction MUI Dynamic Indexing Comparison Credits A Introduction Dynamic indexing is a technique used to ensure that your spell or code can be ran multiple times without any MUI issues. At first, it is incredibly daunting to look at. This tutorial aims to show it in a better...
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However, if you want to commit to putting your map in Lua mode (can't use JASS) then there are some cool resources to help automatically clean up things like this or make it seem simpler to manage them.
Also be aware that all sorts of crazy custom UI stuff is now possible, so old limits on what you can do there basically don't apply.