You need to mesh the slab into a number of plates. Along the left side of the STAAD.Pro screen, look for the Geometry page. Under that, there is a Parametric models page through which you can generate a mesh for a slab defined through any number of nodes. Study the description of this feature in the online help (Help-Contents-GUI Manual).
In the Getting Started manual, take a look at Tutorial 3. It too shows various ways to mesh simple slabs.
In cases like this, you are taking upon yourself the responsibility of meshing the slab into plate elements. If you want to avoid the chore of meshing the slab on your own, surfaces is an option. Surfaces are objects where the meshing is done by the program behind the scenes and you hence have to deal with a limited number of entities on the screen, thus making the model smaller. Surfaces are especially useful if you are modeling a designing concrete shear walls.
However, in complex structural arrangements, this "behind the scenes" meshing may not always be free of modeling errors. You may run into poor conectivities between the slab entities and the beam entities and fixing them won't be easy. Also, if the parameters for defining the surface are not provided correctly, you can end up with a very dense plate element mesh with very small sized elements which will make the resulting analytical model very large.
So, stick to meshing the slab on your own until you are comfortable using STAAD.