Vladimir,

1 - Create a new level and give it a negative elevation, e.g. -2" or -60cm.
2 - Search the Sketchup Warehouse to find models for your stairs. Use filetype:zip to find models that can be imported directly, otherwise use Sketchup to transform the models.
3 - Roofs are tricky business.
4 - Floors will only show when you have defined rooms. Read the User's guide.
5 - Use flat boxes and triangles to make a countertop for your kitchen. Use exact X,Y coordinates for a perfect fit. If you have a rounded corner, make a new level and try to make a room that fits as a countertop. Draw temporary walls, fill them with a room, then delete the walls again.

[url=mailto:[email protected]]Send me[/url] your design so I can have a look at it.

Hans