The first chance to get this is when you reach the world map. There is a chocobo farm on the east side of the map (press options on the controller to make the map larger, it is the furthest East point of the area you are in when you leave Midgar). When you reach this farm talk to the Chocobo (big bird) and say Wark to it. After they dance you will obtain the summon materia. Equip it to anyone and use the Summon option in a battle to get this trophy