A friend of mine wanted me to make a banner for a YouTube channel he's setting up (the channel itself is not done yet). This is the result; how can I make it better?

I think for a banner of any kind the most important is readability. The text "Gray Tunes" is hard to read, because of the rotation, the high contrast blocks it consists of, the weird "R", and the little spacing around it and between the two lines of text.
Other iconic parts should probably be the little character, the drumkit, etc, but they are nearly lost within the high contrasting floor pattern.

What is the channel about? Music stuff it seems? Make the music stuff the focus of the image by making them large and easy to read.

Depends on what you want to do. You could modify the room to have a less distracting floor pattern, larger walls so there is space for a larger logo text, etc. You could take the drum kit and other attributes and scale them up to 2x or 3x.
But most effective will be if you consider your design from the start, sketch out the main elements and try out different compositions of them, and then bit by bit fill in the rest.

Quick (way too quick) edit showing what it could be: