What's the best way to start making games?

Go with unity. If you want to start making games asap check out some visual scripting assets like playmaker, similar to drag and drop gamemaker only way more powerful and intuitive. Also since you are not an artist you can make heavy use of the unity asset store till you get things rolling. Thanks for the tip. The asset store is amazing.

How difficult is it to learn GameMaker?

To answer your title question: approximately as difficult as it is with GM. Go with the one that looks more intuitive to you. So maybe my perception is wrong, but it appears as though GameMaker is easier to pick up, but is limited in what it can do. Unity seems to be a more professional game engine, but has a much steeper learning curve.

Is Unity better than GameMaker?

GameMaker and Unity are both good tools to use. Unity has the advantage of being more modern whereas GM is largely the same tool it's been for a long time. To answer the questions in your final paragraph: either one will work for the near-term. There's been plenty of great games made with both and you can't go wrong either way.