>>18336492mixing, post-processing and sound design is probably where you should start, cuz im assuming you already know the basics of theory from playing instruments.
<for retro stuff you want to stick with basic wave tables (white noise, square, sine, saw, triangle), also bitcrushing and downsampling is good. that's all i got, look up some of the stuff i mentioned, there's heaps of stuff online especially 'tube.