The next mission awaits you by the docks. You can get there with fast travel by clicking on the proper point of the map.
Meet with Achilles and return to the manor after a short cinematic.
Your master will teach you how to manage the manor and send convoys. Start off by buying the needed wood and afterwards choose "Trading". There you will have to choose the goods which you want to send, type of convoy and the buyer. Send out the convoy to finish the mission. More information on the Homestead can be found in the proper part of the guide.