Early game is where the game CAN be won or lost.  Sins is no different from any other game in that regard.  Often times upon reviewing replays, you'll find that even if a decisive blow isn't scored early on, there was some basic advantage that one player claimed, and he just built on that until the late game.

The first thing you have to do is scout.  Auto-scout is insufficient; I find it leaves too many blindspots and does too much backtracking.  Manual scout until you know every gravity well within two jumps of your home world.  Continue to manual scout at least a few scouts until you've found your opponent and then ensure you know all the planets that lie between the two of you.

You should be developing your strategy as you scout.  Identify important planets, choke points, and pay attention to militia numbers.  You should figure out where to send your capital ships and be developing a plan of attack.  Aside from the obvious, like capital ship, extractors, and colonize planets, your plan should be specific to your situation.  Making the right decisions on the fly is the difference between victory and defeat.

In the vast majority of situations, I don't touch trade ports in the early game, even as TEC.  They're just way too expensive.

As for planet upgrades, the important one is population upgrades.  Planets without population upgrades suffer "underdevelopment tax" which will bleed your treasury dry.  Asteroids need one population upgrade, and full-sized planets need two.  You should buy these upgrades immediately after acquiring the new planet.  Other than that, population upgrades increase the maximum population of a planet, boosting its tax income, which is a pretty decent investment. 

Avoid logistic upgrades if you can.  You're better off trying to colonize more planets and using that logistic space rather than paying for more slots on existing planets.  It's a better use of your time and money.  Tactical upgrades and emergency facilities should be avoided.  Sometimes you do need them, but don't get into habit of building them on a regular basis.

The key to building a powerful economy is planets.  Get lots of planets.  Buy the necessary population upgrades, build extractors, and then move on to capture more planets.  It's easy to be distracted by all the little technologies and structures and other goodies available to you, but the key to managing a solar empire is to be frugal.  Take as much as you can by paying as little as you.  Only later on when you have a strong established empire (or there's nowhere left to expand) should you begin to pursue more expensive options like trade ports.