First you must have a steady income to build ships, IE ryujins, garudas, or hercs
Make sure you have a sum of money incase you might go negative in money making per turn.
You attack or explore to get more planets
Again emphasize the money to explore can be costly if you do not max ur homeworld, if ur terran to commercial
Viators and Viator Mark II are exploration vehicles to get you planets, you can attack to win up to 3 colonies.
I wish you luck and hope this helps
If more help is needed on this issue go here
http://gc.gamestotal.com/i.cfm?popup=intro&file=1
Nice little walkthrough to help you get more planets and also a strong prosperous empire