You must build, to conquer space, buy as many Accumulator Assult Ships and Victory Star Destroyers as you can afford. You can also get some Imperial Star Destroyers and possibly a Interdictor Star Destroyer. Once you have your space forces, focus on your infantry. You may need to wait for more income because of all the space forces, but when you get enough, buy some of the first two things you can (for example, Storm Troopers or Scout Troopers), then a few tanks. This will also work for Galactic Conquest mode.