Try this....
Build military stations near your planets. Be sure to reinforce them so that they give your ships good protection. Then station a couple of ships near each of your planets. DON'T give the ships any standing orders. At the beginning of each move, cycle through your ships and check the surrounding space for enemies. A couple of ships, fortified by a base and usually handle any enemy fleet.
However, it's important that you reasearch the logistics (or trade for the techs) line as far as you can.
This tactic works great for conquest. Follow this strategy to conquer systems.
1) Build about 4 constructors. Make them into a fleet.
2) Build a fleet of about 3 decent fighters.
3) Build a fleet of about 3 troop transports.
4) Set a way point right next to the system you want to conquer and give your three fleets directions to go there.
5) Once you get there, build a military base. Attack the defenders with your fleet of fighers (you should be able beat them with the help of the base.
6) Invade.
Repeat indefinitely for a conquest victory. This takes a long time but trust me, your patience will be rewarded eventually.
