Hi!

What script I'd have to write to when te Pc sleeps in balmora, pop up a messagebox talking about a dream, the dream doesn't repeat if the messagebox was pop up already, and give the topic "old man dream" to the player? thanks!

This should work for, though I didn't test it.
Place the script on a unique bed.

Begin My_Dream_Scriptshort playerSleepif ( playerSleep == 1 )   if ( GetPCSleep == 1 )      set playerSleep to -1; this makes it only happen once      MessageBox "You just had the most disturbing dream.. etc etc etc."	 Addtopic "old man dream"   endifendifif ( menumode == 0 )   if ( OnActivate == 1 )      if ( playerSleep != -1 )      set playerSleep to 1; prevents any other bed in cell from triggering.      endif      ShowRestMenu   endifendifEnd My_Dream_Script

Not sure if you need the bit about triggering other beds, but I seem to remember needing it for one of my scripts.
Better safe than sorry