I tried to subsurf this model that I mirrored but when I use subsurf it splits the model into two objects.
Any help would be appreciated.
A couple of things could be effecting your model.
The order that the modifiers are listed effects how they interact. Having the subsurf before the mirror can cause the edges in the centre of the mesh to pull away before the mirror duplicates the mesh. To fix this you need to have the subsurf under the mirror modifier. You can use the up and down arrows on the modifiers to re-order them.
If your subsurf modifier is after the mirror and the edges still shrink away from the centre of your object you need to enable merge in the mirror modifier. Enabling clipping in the mirror modifier can also help to get the centre vertices aligned so that they merge.
From what your screen shot shows, these are my suggestions : Make sure that 'clipping' is turned on for the mirror modifier on the
y axis, then add smooth shading to the model, and apply the mirror modifier. Then add a subsurf modifier. Apply the subsurf modifier, and all should look good.