It's best to avoid the carve tool when you're just starting out. It's a great tool but needs to be used in the right way, and it's easy at first to get into very bad habits with it.
Build rooms yourself rather than hollowing them out of cubes. Make the floor. Then each wall. Then stick a ceiling on top. It works out much better in the end.
I've made an example map for you. You can find it at the link below. There are no textures in it, just solids.
http://rapidshare.com/files/36316035/room_and_door.zip.html Personally, I don't use hollow at all. The way it creates the walls is pretty bad.
Penguinboy's sweeping generalisation about carving is wrong. Carving can be very useful and save you a bit of time, but you have to be very careful to make sure that when you carve something, it only creates 90 degree angles. If you try to carve something like a cylinder, you're going to end up with very messy, and possibly unusable, brushwork. So, if you're cutting a square doorway through a flat, square wall, then carving is fine. That's what I did in the example map. If you're doing anything more complex than that, then you're better off working out how to build the required shapes yourself.
128 units is a good general height for walls.
I like to make my 'average' doorways 56 units wide and 96 units high. The player is 72 units tall and 32 units wide (if I remember correctly) and 48 tall when crouching.
So, look at that example map (it's incredibly simple) and practise by copying and pasting a few copies of the rooms nearby, and try joining them up with corridors and rooms of your own. You should learn a few things from that.