Some sort of music workstation
It also depends on whether you intend on writing with midi sequencing or if you want to actually write with a synthesizer/keyboard (in which case you would want a midi keyboard and compatible computer)
FL Studio is a popular, cheaper and solid entry-level audio workstation.  Comes with a good amount of samples, you can pay for more or find custom ones people upload on forums.  Drum machines are generally included with some decent samples to get you started
Before you buy anything just look up examples of people's work, tutorials, and try free trials if they're available