Yeah, I kind of used to play like you. Traditional RTS games give you a sort of clustered base building mentality that is not the way to go in Sins. Early on you have to suppress your turtling instincts and try and capture as many neutral planets as possible in the first phase of the game. The more planets you have the larger the economic and research infrastructure you can support. If you just sit back and let other empires get more planets than you do, you're going to be fighting an uphill battle the rest of the game.
As a sort of build order, at the start of the game I'd build a colony capitalship, two light frigates (LF) and a colony frigate, and 1 civic and 1 military lab. Research corvettes right away (unless Vasari, get Assailants instead), and then try and max out your starting fleet supply with corvettes. While you're building them up, send your two LFs to a nearby asteroid or dwarf planet and attack the siege frigate (as you build your first corvettes you might want to send a few of them in too). Once the siege frigate takes some damage send in your colony frigate, hopefully it'll be destroyed by the time it gets there.
Meanwhile, send your colony capitalship to a completely different planet, preferably a weaker Ice or Volcanic, as it can handle these planets unsupported. Take out the siege frigates first. If its an ice or volcanic, you'll need to colonize a roid/dwarf and build a civic lab before you can research the colonization techs (build the lab before extractors there in this case). If there are lots of militia left by the time you colonize it, you don't have to kill them all. Build a basic turret somewhat away from the action, after its mostly complete order your cap to the next planet to take. After it leaves the militia will go attack the turret which will probably kill them (if it can't hit the long range frigates you might need a corvette or two to finish them off). You can then scuttle the turret latter if you need extra resources.
After this it depends greatly on the map, but basically you want your two groups to go and take as many neutral planets as possible. Try to expand towards choke points with other Empires as well, and when you eventually hit enemy worlds or a serious attack is sent against you, only then should you start expanding your fleet and focus on things besides expansion, trade being a major one. Hopefully with this you should have about as many planets as anyone else, and from here you'll have the breathing room to play however you like: attack, turtle and research/develop, etc.
