income comes from various sources and for some races more than one or in higher lower rates in one or another
to check your race`s stats, read the manual, and go down to encyclopedia, under which says "races" click that then your race and check out the modifiers at the bottom
you can earn income from tax from your population, from selling food on the market, selling ore on the market, selling mninerlas and other goods in the market, selling goods to your population, building commercial(they just arbitrarily make money on their own per-turn)
for terrans, focusing on commercial will work best