Bullfrogs are big, active frogs with long jumps. They really should have a very large enclosure, much larger than the typical home aquarium. Even in a large enclosure, expect them to be jumping against the glass.
They also have huge appetites. They will eat massive quantities of insects, and will even eat mice.
A bullfrog is a poor choice for a first frog. My advice would be to release it where you found it.