According to his page on Wookieepedia, Admiral Ackbar was a decorated officer on his homeworld of Mon Calamari, leading them in many battles and later leading a resistance movement against the Empire.
Ackbar was selected as Coral City's representative on the Calamarian Council after finishing school, and became a proponent of Mon Calamari space exploration during the Clone Wars.
Ackbar was one of the leaders that participated in the Second Battle of Mon Calamari. During that time, he held the rank of Captain in charge of the Mon Calamari Guard and was the chief military adviser to King Yos Kolina and then to the young and inexperienced Prince Lee-Char.
In 19 BBY, Ackbar was chosen as the Calamarian Council's leader. When Imperial warships arrived to exert Emperor Palpatine's will over Mon Calamari, Ackbar tried in vain to negotiate; the Mon Calamari and Quarren were forced into slavery and the orbital shipyards were nationalized. Ackbar quickly became a leader of the resistance movement and succeeded in temporarily freeing his homeworld.
It seems that he had much experience rallying troops and fighting against oppressive, overwhelming opposition, as well as inside knowledge of the Empire from his time as an Imperial slave.