Master of Orion: Conquer the Stars/Expansion

Basic Colonization Expansion
You build up a fleet of the strongest war ships available. Example: 2 or 3 Frigates or Destroyers

Then you send the fleet to an unclaimed system - typically adjacent to your territory. Send each ship to an outbound warp point and set them into guard mode.

Afterwards, send a Colony Ship and Space Factory to the guarded system. Colonize the best planet. Get the Space Factory to build a Military Outpost at each guarded warp point.

Once the outbound warp points are protected by a Military Outpost, gather up your war fleet and a new Colony Ship and repeat.

If the lane to this system is protected at the last system, scrap the Military Outpost as it will no longer be needed.

Some Suggested Colonized System Structures
For a colonized system which has an asteroid belt and / or a gas giant planet (while the Space Factory is in the system):
 * If the system has only one of these options, build credit generating structure (like an Asteroid Mine or a Superscaler Gas Harvester)
 * If both options are available in the same system, build the Superscaler Gas Harvester (for credit) and an Asteroid Laboratory (for research points)
 * If the system has two asteroid belts, build an Asteroid Mine (for credit) and an Asteroid Laboratory (for research points).

Blockading
As your Scout ships map out the various planetary and wormhole systems of the galaxy map, make note if you encounter other opponents. Using the Basic Colonization, claim the systems, one planet at a time, with the objective to block expansion by the opponent. Certain races are expansive, notably Meklar, Klackon, Sakkra. If you can block these races before they can expand too much, then controlling the diplomatic exchanges will be easier. As an extra bonus, if they happen to have an Antaran asteroid system, you can seize control of said system to help achieve the Antaran Victory condition.

Use the Pirates
As you expand your empire territory, note the system which the inbound harassing pirate fleets originate from. A Military Outpost is sufficient to keep pirates out of your systems. Pirates bases tend to start up in unclaimed systems which happen to lay close to your empire. As long as the pirates keep coming, you know that their base system has not been claimed by an opposing race. Send a fleet with 2 or more bombs available to the pirate base system. Bomb the pirate base and then guard the outbound warp points.

Scorched Earth Policy
When you make war with a weak opposing race, the scorched earth policy is usually the best if they have resources which you need and you want a rapid conquest. Basically, bomb their planets and eliminate the existing population. Have a Colony Ship ready to settle the system along with a Space Factory to build defenses. When they diplomatically contact you to try to end the war, you know that your Military has the upper edge. Decline them and continue to take their systems.