Pick a race and learn it well, as each race has certain build orders that influence race strength and weakness.
Since you are on the forums, spend a good time reading about the different capital ships and frigates. Knowing when to run from 10 ships and laugh at 50 ships is important.
INTELIGENCE:
Scouts are your best friend. They tell you when to attack and when to pull back.The main thing is tempo, keep attacking, make sure you have scouts flying back and forth in the direction of the enemy. This way you can attack empty systems even if its just with 2 ships. These should be the first thing you build so you can work out whats around, how far away stuff is, AND how you are going to spend the next few minutes expanding.
REAL ESTATE: Location Location Location
If you have scouted quickly at start and have a 4-5 minimum scouts moving around, then you can look and see how to expand. The planets next to you might not be the best first choice. If the systems next to you are ice or volcano consider jumping straight past, especially if there is 3 asteroids in a row beyond. At the start of the game, I usually cap the nearest roid with a colonsier, the nearest planet with the cap ship. This way you are attacking two systems at once. The other thing is to MAKE SURE you upgrade all your planets quickly.
One capital ship can handle 4 enemy ships no problem. Send em in, take out the seige frigates, and then cap the planet. This way you own the planet even while you are taking on the fleet.
You dont need a whole fleet of ships to capture an asteroid, only one coloniser. Send it in, let it capture the roid, then build a defence turret up against the roid orbit (as close to the roid as you can) and build it in front of the now approaching seige frigate. Upgrade the planet, send coloniser out towards next roid, or bunch of neutrals. If you dont have enough anti-matter when you jump into the system, just hold down shift and click around the roid a bit so that it will keep moving to avoid the ships there.
FREE SHIT:
Go for neutral mines/systems as much as possible - These can be a game winner. Again, building coloniser early and getting them out taking down neutrals and roids is
FLEETS:
Research is one thing, however nothing influences the game more than fleets.Get one, keep it big. Then build another one somewhere else. Having multiple fleets moving around, one to attack enemy, one to cap all the spare planets. Fleets are worth so much more than defence cos they move.
SHIPS > STATIC DEFENCE
The main thing to remember is QUANTITY has a QUALITY all of its own. 
TRADE:
Maintaining a good trade/military balance is crucial in sins, like most RTS. At the start, dont worry so much about trade, just grow your fleet and capture at least 4-5 systems. You want to be scouting your enemy, moving towards them, and capturing a nearby planet or roid to build factories. Once you have established a good front line and are attacking you enemy take moments whilst moving your fleet to simply start building trade behind you. As long as you are attacking and capturing systems you keep making more money.
or the simplest easiest way to learn?
Down load and watch a few multiplayer demos, watch how people expand and grow really quickly. You can also watch what they research and how they control their ships in battle. I STRONGLY recommend you watch a few battles, learn to smash ai, then go teach your buddy some new tricks
