I want to start making new enemies in the Hello Mario Engine, but...

[Pedigree]

Thankfully, Hello Engine uses very basic code, so I think you'll have less trouble understanding how this engine works.

I feel like knowledge of GML could really help me. But I don't know what tutorial teaches the style of GML that the Hello Engine uses.

There's many tutorials on YouTube as Super Sledge Bro said, there's also paid classes that teach you GML by having you code a game step by step. Unfortunately, you'll have to do your own legwork to find the one that's right for you.