Thank you very much!
Your best resource to start I reckon is the GBS docs themselves, they're not the most enlightening thing ever, but they'll help, and once you know enough to wanna try some more advanced stuff, you should check out GB Studio Lab, and if you have trouble, the GB Studio discord is mostly pretty helpful!
Though to bear in mind, A LOT of tutorials, particularly in video form, are Outdated As Heck...
Oh and like, being knowledgeable about how the GB Hardware works helps a lot, so by all means, study that as well, this here's a VERY good resource on that: https://www.copetti.org/writings/consoles/game-boy/