Select the menu item Solve -> Load PDE and open the PDE file ngsolve/pde_tutorial/d1_square.pde. You get some text in the shell window, and you will find a blue square in the black area. You have loaded the problem description, including geometry and mesh. In the button line (below the menu items) you see the selection-button showing 'Geometry'. Here, you can switch to 'Mesh', and later, you switch to 'Solution'. This selects the visualization mode in the black area.
Now, you are ready to start the first solve. Press Solve -> Solve PDE. The visualization switches to Solution, and you see two gray triangles.
Click Solve->Visualization to open the visualization dialog box. Here, you can select what you want to see. Look for the selection button Scalar Function, and change from 'None' to 'u'. The triangles will be colored.
Clearly, this mesh is too coarse. Go on, and press Solve -> Solve PDE five more times. The mesh gets refined, and new solutions will be computed.
Play around with the items in the visualization box. You may try Autoscale, Isolines, and Textures. Go on and do some more refinement steps (by pressing the short-cuts <s>-<p>.
The next chapter will describe the structure of the PDE input file.