I assume you are a Terran. So here is a basic Commercial Income plan to start out.
->Economic -> Research -> Commercial (Click/Research it till it becomes Level 20)
Then click "Manage Colonies" ->click "E.1371"
(Something that looks like a bunch of numbers together)
Built Housing, Agriculture and Commercials by typing in numbers in the boxes and pressing "Built"
a. This allows you to built more things on your planet (housing)
b. This produces more Food for you (agriculture)
c. This produces more Money for you (commercial)
You would thus want to have a proportional amount of each of them.
(You would want to have just enough housing to fill your planet ; just enough agriculture so that you make a decent amount of food per turn and focus the rest to commercial)
Exploring is a good way to increase your total land to build more Commercial.
Click "Build Ships"-> Click "Viator" ->Built 25 of them
Then click "Explore" and start exploring for 6 planets now you got new planets to work with do the same and built Housing, Agriculture and commercial on them.
(Warning: Remember to click "Manage Fleet" and Disband/Destroy all your ships after you found the planets by typing the amount of ships you have in the boxes and then press "confirm")