These tips apply to all RTS games equally. I am not a sins expert, but I do well online because this is not my first RTS game experience:
The biggest problems I see in noobs are not bothering to scout, not making fleets, and not using money wiseley. These are things that seem obvious when you look back on things, but are not always obviously important at first.
The first thing you need to do in any game is build at least 3 scouts and set them to explore. You need to know where to make your goals for expansion (finding that one key choke point before they do...), and what your enemies are doing. In this game, there is no real excuse not to keep close tabs on your enemies all the time since scouts are so cheap. You should even go so far as to set your scouts to move back and forth across your enemies' planets, using the "hold shift while giving multiple orders to chain commands" trick.
More important even than that, though, is that people don't build big fleets. If you have a small fleet, and your enemy has a big fleet, you just lost. Simple as that. Economy, research, culture, colonization --they are all secondary to the ability to crush your enemy's fleet, because once you are bombing their homeworld, it's not going to matter who has 5% more laser research. The only reason to focus on anything other than fleet strength is if you have already thoroughly scouted, and you KNOW FOR SURE, that the enemy is not a threat yet. (one caveat: It may be adviseable to skip light frigates and go straight to LRMs, since they are much better in the early game. Therefore, your first goal should be to get a sizeable fleet of LRM. Like everyone else has said, defenses are not that useful, since the enemy can just go around them and attack a different spot)
In the case that you have scouted and the coast looks clear, it makes sense to build an economy, expand, etc. In fact, I would even go so far as to say in that case it makes sense that you would WANT a smaller fleet, since that means you have more resources for expansion than the other guys. As you can see, either strategy completely relies on your knowledge of what is going on elsewhere in the solar system.
One last thing that is super important: Don't stockpile money (of any kind)! There are no high-yield savings accounts in sins! If you have more than you can spend, you are not spending enough, and you are falling behind, because you can bet your enemy isn't just sitting there uselessly counting his money.
