You get taxes from your population so increasing your population increases your taxes.
Also increasing loyalty on a planet increases the taxes you receive on that planet by 0.5% per point of loyalty (loyalty maxes at 5000 but you can also receive bonuses from missions after that).
However as a terran you should focus on either commercial or agriculture as those are the most profitable for your race. Going tax as terran is far less efficient and since taxers have a high population it raises your base power rating making it harder to fight any where but the upper ranks.