How do you invade a planet???

[Mako]

To invade, simply attack the planet with your transports, move them next too the planet, and use the orders button or the guick key T and the troops will be droped and ground combat will begin.
Send warships along for protection and to supress ground defenses. You can take out the weapons platforms by turning off the high damage weapons and fireing on the planet until they're gone. This might be harder on smaller population planets, but it works.