Frist off, your map has to be collidable using the collision tags {Polygon descend keep}.
If you have done this step, then go on adding your collision to the camera using this code.
#Setup collision manager
base.cTrav = CollisionTraverser()
base.cTrav.setRespectPrevTransform(True)
#Pushes objects back
base.pusher = CollisionHandlerPusher
#Add a collision sphere node around our camera.
self.cnodePath = base.camera.attachNewNode(CollisionNode('CollisionSphere'))
#Setup the collision sphere.
self.csphere = CollisionSphere(0, 0, 0, 4)#May need to change this number as you need.
self.cnodePath.node().addSolid(self.csphere)
self.cnodePath.node().setFromCollideMask(BitMask32.bit(1))
self.cnodePath.node().setIntoCollideMask(BitMask32.allOff())
#Add our collision node to the puser and then to the collision manager
base.pusher.addCollider(self.cnodePath,base.camera,base.drive.node())
base.cTrav.addCollider(self.cnodePath,base.pusher)