Articy Draft is supposed to be very good:
you can use mindmap software there's quite a few free ones that are useful, I've heard that some people use text game engines like Twine
Or good old pen and paper!
You can use place holder art and change it any time. likewise music... music can drive you loopy there's going to be a lot of build, test, fix,test, fix, break, pull hair out moments and repetive music isn't good lol
Just have fun, I personally just run with ideas and do as I go, I lose interest if I try and plan.
Entering contests are a great way to learn, often have themes and a time limit