I am a new Fluent user. I have imported a mesh from ANSYS into Fluent. But I the mesh has only two zones: exterior and interior. The 2-D mesh is a cirle in a square. I need to apply boundary conditions to each wall of the square, and to the boundary of the circle. I need to simulate flow over a cylinder in 2-D where I have a top and bottom wall, a flow entrance and exit. How do I manipulate my grid, to obtain surfaces and zones that will allow me to place these boundary conditions.

The boundaries of Your problem (inlet, outlet, walls) are grouped together in the exterior zone. You have to split this zone (grid/seperate/faces) and to change the boundary zone types (define/boundary conditions).

Yes, but how do I proceed from grid/separate/faces? Do I need to learn how to apply marks and separate that way? I am having trouble from the point you recommended.

Yes, I think so! Try the feature "mark" in order to separate the square and the circle. You can apply marks via adapt/region. Then divide up the different zones of the square using "angle"