No problem. Since you don't have a guide, here are the GFs to look for to draw from bosses:

Siren-Boss at the Tower
(SPOILER)Carbuncle-The lizards in Deling City
Leviathan-NORG
Pandemona-Fujin in Balamb
Alexander-Edea in Galbadia Garden
Eden-Ultima Weapon

Oh, and did you get the Magic Lamp from Cid before you went to Timber? You need it to summon Diablos.