Some good tips for you in this thread already, so I don't have very much to add

To increase credit income considerably, build a few trade centers - they'll help a lot! Especially as TEC, you can get them very early. Even 3 of them on 3 planets (1 per) will give you a big credit boost.
Upgrade your homeworld to level 4 pop as soon as you can! And do those Terran population researches, it's also a huge help.
When you move up the tech tree a bit, grab a broadcast center. Culture spread increases planet allegiance, which in turn increases credit/resource income. Just build one on your homeworld, and you don't need to do anything else, just get to watch more money come in