Okay,
now's the time to load up some floor textures and we
do this by going to Textures>gothic_floor.
Scroll through the available textures until you find
the one called metalbridge06, this is what we'll use
for the floor.
Un
select any objects that may be selected by pressing
escape and now with the new texture loaded (it'll
have a red box around it) draw out a new brush until
it touches all sides of the walls

Now
we need to make sure that the floor brush is where
it should be, check in any of the side views to see
if the brush is level with the lowest point of a
wall.

You
getting the hang of it yet?, repeat the same process
for the ceiling as you did for the floor but this
time you need to make sure the the ceiling brush is
at the top of the wall.
Okay,
thats your first ever room, but before you can try
it out we need to place a couple more things in so
it'll work.
Somewhere
in the grid right click and a drop down menu will
appear, scroll down to info > then that will open
up allowing you to select info_player_start.
For
your level to work okay you need to make sure this
item is not stuck in the floor, do what you did with
the floor brush and place it so it's about 32 units
above the ground, this rule is the same for weapons
and items, when the game starts they will already on
the ground.
Next
we will place a light entity in the map, do as you
just did but this time select light from the drop
down list.
now
place your light entity so it's in the middle of
your map as well as about 64untis of the ground.
Save
your level and then from the BSP menu select
bsp_fullvis.
A
dos box will appear and if all is well it will start
to build your level, once it has finished compiling
close that box and if you have the option to run
game after qbsp3 in the preferences then quake 3 will
start and automatically put you in your new level :)
pretty
easy really