Everyone has a different idea on what to start with. I like to start with the LOD terrain (keep it small for your first map), and then add a skybox around it. Make the bottom sky brush caulk texture. It should look like this from below the terrain.
This one has tunnels.
Making the terrain look good is not an easy task and almost an artform in itself. Make a simple lake, pond, river, or canal depression in the terrain. Now you can add your water brush to fill it.
Next comes roads which are made from simple patch mesh. Also, add your tree line around the sides of the map--can be solid brushes with nodraw on all sides but one, or patch mesh. Add a few tree and bush models, but don't go crazy with them. Finally make your buildings, walls, and other structures.
Don't forget to add light to worldspawn and player spawn points.
Hope that helps!
BTW, this map was never released because of bad frame rate. It is to big and open.