In GameMaker studio 2 program. Help heading in top bar click on "open manual"
Watching the GM tutorials videos is a good way to start for most people. They have some new ones that I liked. Shaun Spaldings were good too. Lots on you tube.
Udemy has some courses as well.
Some hire mentors/ teachers.
I think most people start with the videos. Everyone learns differently so whatever works best for you.
Happy learning. Have fun.