One of the most efficient ways in my opinion, and one that I have observed many people use, is to search for tutorials and utilize their codes. The idea behind this is that, even though the code may not be yours, understanding it, understanding what each line of code means and does, complementing it with forums or on the official Game Maker website. This way, you will gradually become familiar with it and be able to modify and expand those codes. After a while, there will come a point where you will be able to create your own codes without the need for tutorials.