Look up
audio trackers. They're pretty low-level. First you construct instruments from your own sound samples, although many include samples and some can use your MIDI sequencing subsystem for additional instruments. Then you place notes, with stops to end sustained notes when there's no new note following them, onto a top-down timeline of beats which are grouped into pages called patterns. Each pattern can be repeated multiple times, too.
If you don't have a MIDI keyboard and can't play piano-like instruments, a tracker gives you the ability to construct music on a simple, technical level using just your computer keyboard and a bit of patience. Also, since they've been around for a while, there's a lot of free ones.
Just don't let yourself be scared off by the horrendous user interfaces, and you'll be fine.