Now that you have downloaded, installed and started GtkRadiant, you will probably start to draw around a bit with your mouse, and you will see that where ever you draw on the grid, a red square will appear, now you may think, is this really the program all others made those awesome maps in, the answer is probably yes, so, how is this done, first of all, you have to learn the interface.
This is the 2D Viewport, in here you can either see your map from above, from the front, or from the right. To change the view, press CTRL+TAB or click the button in the toolbar. This is also where you create brushes, to create a brush, just drag with your mouse.
This is the 3D Viewport, in this view you can see your created map in 3D, similar to what it would look in game. To navigate in the window, right click anywhere in the window and you can look around by moving the mouse. You can also use the mouse wheel to scroll back and forth as well as the arrow keys to strafe in any direction.
This is the Texture Browser with which you browse and choose the texture(s) which you can apply to the geometry. On the left side you see a list of folders from which you can choose the textures, double click a folder to display its content on the right side of the panel.
This is the Console where GtkRadiant prints down every major (or minor) thing it's doing. This is very useful if you get error messages or if something else doesn't work like it should. Note that all critical stuff is displayed in red color.