This will teach you about how to make music for Orbiter, using a freeware called Modplug Tracker (MPT) which can be downloaded from www.modplug.com
If you have problems to locate the right files, enter the forum and there is a special forum there dedicated to downloads with the right links.
This tutorial was made for the old version of MPT, but it is useful for the new OpenMPT version too. The old version has less features. Newbies may not seize the advantages of those new features, so if you want a simpler user interfase, you may download the old version.
In the tutorial, a software called Sound Engine is mentioned. English freeware version seems not to be available anymore, but you can google for "Audacity" which is an opensource program that is useful for mastering and EQ too.