You just drag a new brush on top of a corridor and texture the bottom side like so:
You can switch perspective in the 2D view by pressing Ctrl + Tab. Move the brush where it belongs and you're pretty much done.
You can also texture individual faces by pressing Ctrl + Shift + Middle Mouse Button. It will use the currently selected texture, marked in red in the texture browser.
I'm not sure how you're working right now but I'm doing my brushwork in the 2D view and the texture related stuff in 3D - works best for me.
Before actually working on a real layout/map, you might want to start out with something smaller like our weekly challenges (viewtopic.php?f=26&t=502
) - it's really annoying to finish half of the map just to see you messed up the layout or made some mapping errors in some way. (I'm still busy redoing most of my map (~4.5k brushes) because I screwed up on nearly every wall.
For now I think it's best you do some small things to get the hang of Radiant and mapping in general - you'll have enough time worrying about layout later on
You're welcome on IRC, there's someone there almost all the time and it's probably easier to help you there instead of writing here.