[sixhobbits]

the github links to a bunch of other places that also pay. There is also whopaystechnicalwriters.com which has a link to a book that Philip recently published with more thorough advice.

otherwise write, write, write. There exists no method that will make you a better writer than writing a million words. (loosely paraphrasing patio11). Once you have published something it is much easier to find follow on clients. Publishers like PacktPub, Apress etc will usually happily take on inexperienced writers. They don't pay that well but they'll push you hard to get words out the door.