1. When I started to draw the top level, how do I add one below?

Double-click the tab on the new level and insert the correct elevations/floor thickness and ceiling height (for each level).

2. How can I create a roof?

If you look ah the FAQ and in the forum you will find hints, e.g. thread 6372 has some links to start you off.

3. How can I creat a wall which starts at the sealing(if you have a room which a part of it is sloped)?

If you modify a wall, it is a question of setting start and end heights. Often you will first need to split the wall first. ok