After you plant, make farm , metal and oil structures to level 3

Then make your barracks..and from order menu train troops..each kind of troop has its own time to get ready..once it is done, move them all to a squad , select the squad and click on the destination ( u can only select target inside the Green Circle i.e 24 ticks from your colony)

Then click launch and ur squads will be flying...Try doing the in game missions....it will show u how to attack and pull back attacking squads