The first step is coming up with a few good sounding instruments.
A good starting point is thinking about the properties, sounds and modes of playing a physical instrument, and then try to create a simplified abstraction of those properties.
The london philharmonic orchestra has a youtube series (and an interactive app) where you get introductions to each common instrument in an orchestra. The string ones are very basic introductions, but some of the other instruments go into sufficient detail.
Composing music takes practice, practice and practice. And theory. You'll come a long way getting comfortable with repeated combinations of the different note lengths
(rythms) and practicing moving through the circle of fifths