There are TONS of threads about this, if you search the forums you will find lots of valuble info.
Basics:
The more trade posts you have (one at each planet in an unbroken line) the more you make. You can see your longest trade chain by hovering the mouse over the credits in the HUD, and a white line will appear between your planets showing you your chain.
You dont HAVE to have a chain to get benefits from trade posts, but the longer your chain the MORE you make.
You will always make like .02 (or something...?) off a trade port regardless.
Thats how I understand it all anyways, so find a thread already posted about this, there is prob more info there.