Why don't you sign a peace treaty and behave as a good boy?
For taking planets, build transports using the fastest hull you have, add some armor and a lot of shields, then load your troops and go on the rampage.
If instead of conquering the planet you just want to glass it, and if you have researched late level of missile tech you can fire barrages of missiles from far away. This missiles will be intercepted with Point Defense Cannons, but it will entertain the Point Defenses while your fighters or drones charge the planet.