Here are some random early game tips of the top of my head:
* Try not to research stuff until you need it. Sitting on 5000 Knowledge isn't bad; it means you have the resources to pull out some new tech when you encounter an enemy system that otherwise would stop you. If you run your Knowledge down to 0 constantly, you'll soon find yourself in the tough situation of not having the tools you need and no where to expand to get more Knowledge.
* Unlock Gravity Turrets immediately. Place at least two to cover each warp point in your home system, between the warp point and your command station (form a triangle with the warp point being the third point). These turrets are so amazingly useful. In fact, they are basically the most OP turret in the game. If you've ever played a tower defense game you'll know slowing turrets are generally pretty awesome. Now imagine a slow-aura turret in a tower defense. Yup, they are that good.
* Fighters are really strong all-around ships. They have a lot of survivability so they make a really strong early unlock, especially if you find yourself needing to defend a lot. Bombers are awesome on offense and a great unlock if you need to take enemy systems quickly. Missile Frigates are...less impressive. Try and avoid unlocking them unless they are a hard counter for one of the enemy's ship types.
* Early game you probably don't need to build any additional Space Docks or many (if any) extra Engineers. Your resources won't be able to keep up with insane build rates so you are just wasting resources creating super-fast building potential that your resource base can't support. A good early-to-mid guide is one Space Dock per Mark of ships. So once you've unlocked some Mark II ships, build a second Space Dock. Don't build a third until you start unlocking Mark III. Unless you know what you are doing, don't build any extra Starship Constructors or Merc. Space Docks.
* Initially your main goal should be to take all adjacent systems. While building up your fleet ships to do this, scout all those systems and everything adjacent to them. You want to be very careful about taking a system next to a Mark III or worse, Mark IV enemy. You absolutely can crush a Mark III planet 20 minutes into the game with just one or two Mark II unlocks. However, that only works if they aren't on alert very long. So early on if you do have a nearby Mark III, try and take it right after you put it on alert. Do this by clearing out the adjacent system of everything except its Command Station and then rebuild your forces. Then drop the the Command Station and go straight for the Mark III. You don't need to take it in one shot, but crush as many Guard Posts as possible (ignore the Wormhole Guard Posts).
* Until you start capturing enemy system, don't unlock Economic Command Stations. I like to wait until I have 2-3 systems claimed first. Basically everything adjacent to my home world. Don't improve harvesters unless Economic Command Stations aren't cutting it which should be rare.
* If you are generating too many resources early, build up some starships. There is nothing wrong with pausing starship construction often. When my resource reserves are climbing, I unpause. When I start losing reserves, I pause. Eventually you'll get some starships out. Raid, Bomber and Light Starships are all extremely useful early. With experience you'll probably want the Raid Starships first because they are such a great tool for removing problem enemy structures such as Warp Gates, Ion Cannons, etc. If you don't think you need those, try Bomber Starships. They really help speed up taking enemy systems. Just make sure to pull them out before they die. You'd much rather repair than replace them. Light Starships are a touch fragile, so while they multiply your power, they can be tough to keep alive.