if ure terren build commercial on ure planets and spend time researching commercial. if u`ve run out of land try exploring for more planets. aim to try and keep at first planets which are 600land + as the more planets u have the harder it is to explore. also be carefull exploreing more planets raises ure pr. if u go over 5k pr u will be open to attack from other empires.
if ure a miner just research mining and sell the minerals that u produce onto the market.
(this is just a basic guide lines as each race has more than 1 way of making money)