A few months ago i raced online, they used Fernbay rallycross track, with an extra jokerlap.
How can i rebuild tracks?
Create extra chicanes, make a rallycross track from a race track, by add some corners trough the gras...

Is there a "how to" somewhere?

Press Shift and U and then from lower right corner press edit.

In wich screen i should push shift and U?
Nothing happens when i push these buttons...

ingame when you can drive.

If i press shift+U nothing happens, only the view changes...
Do i use it off line or online?
Should it work on all tracks?

After pressing Shift+U, you see a button interface. Click "edit" and by pressing O you can add objects.

Check the lfsmanual, everything you may need is in there.

http://en.lfsmanual.net/wiki/Autocross

Sometimes you do not see a button interface.

try shift-U and then hit the spacebar.

okay, the space bar trick works!

Next problem:
If i drive my new build track in ofline mode, all the AI cars crashes on the new track parts...
How can i solve this?

Not possible. Race lines do not exist in open track mode.

aha, so this problem will stay