You're going to need some kind of flash cartridge. The cheapest is the 64mb Smart Card but those can be annoying to put ROMs on, but there are a lot of other options.
Then you're going to need some kind of program to write music. I like LSDJ because you have a lot of control over every aspect of the sound but that can be daunting and some people like the simplicity of nanoloop 1. Another option is to get an Arduinoboy and MGB, this allows you to use your gameboy as a midi synthesizer to be controlled by a laptop or keyboard, but that's a little more expensive.
Other than that you just need to be persistent to get used to the interface of whatever it is you're using to make music.
