Selection is one of the first things that you should know about Radiant. Below is a list of my favorite selection methods and how to use them.
First off you have 3 basic ways of selecting things in Radiant. Selecting
from the Edit/Entity_Info..., Selecting from the 2D views, and selecting
from the 3D view.
You can also create a new brush and use these buttons.
I will list them from left to right, 1 2 3 4.
1. Complete Tall - Make a brush on the screen and everything inside of it (in the 2D view) will get selected once you push this button.
2. Select Touching - Make a brush on the screen and anything it touches in the X, Y and Z views will get selected, your brush will turn into part of that geometry also.
3. Select Partial Tall - Make a brush on the screen and anything that it is touching in the 2D view will get selected.
4. Select Inside - Make a brush and everything inside it (counting the Z axis) will get selected.
Selecting from the 3D view - When you are looking in the perspective view you can also select objects by holding your shift button and left clicking on them. If you want to select a face of an object just hold Ctrl and Shift then left click on the face you want to select.
To apply a texture to a face or object that is selected - either left click on the texture it in the textures window or, if it is already in the scene click on the face of the object with the texture you want with your middle mouse button (must have a 3 button mouse) and the selected object or face will have that texture.
If you want to apply the texture of the selected face to other objects just hold Ctrl and Shift and middle click on the faces that you want to have that texture.
If you want to select a texture that is in your map, from the 3D view just middle click on the face with the texture you want selected and in the textures window, and that texture will get selected.
This is all kind of hard to explain so all I can say is just open up the Radiant program and follow along with these tutorials. That is the only way you can really understand, is if you follow along.