You start out as a terran so here is how it works:
You get planets by fighting or exploring. You populate them by building housing on the planet. Then you assign the population to commercial (or agriculture), then you start increasing your income by doing infra research in commercial (or agri). If you need some starting cash it would be wise to extract all ore from planets (assign some people at mining for the period it has ore). You sell that for 10K each and you can make quite some money from that. Be active, get nice planets and do tons of research ;)