you should start with the independent planets that just have defenders as their defense fleet. zeus and hera are very easy to take over, so that is a good place to start. defenders are ideal because they die in one hit from a torpedo, so sell everything on your ship and buy a launcher and stock up with a ton of torps. as long as you stay somewhat distant from the planet you should be able to take out the defenders before they pose any real threat.
from there you can go on to other independent planets with more defenders, and to the fringe confed/rebel planets that just have mantas or patrol ships and gunboats. sometime along the way youll want to pick up a rebel or confed cruiser. with your cruiser of choice you should be able to move on to dominate most of the planets in the galaxy, although a few will still present a problem -- particularly those in the sol system, and ruby. they have over 32000 defenders each, so you really dont stand much of a chance. if you are determined to take them out, use weapons that dont require ammo (ie turrets) and good luck.
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-Asmodaethur