Make sure you have researched planetary invasion and a few more of the later techs.
1) find planet that has enough people to fill your specific transport.
2) Make sure planet can produce the transport fairly quikly.
3) Launch transport and fill it completely.
4) Make sure enemy planet has no fighters orbiting it.
5) Invade (send transport to planet).
I viewed the tutorials but yet I can't understand how to do things, such as counquer a planet
you said such as... is there anything else you don't understand?
because we'd be happy to help.