You have multiple options, of which writing novels is the one that can make you very 
rich, very fast:

1.Start writing novels on your computer. Write fiction novels as they take the least time.
  They will not bring you too much cash.
2.When you write your skill will automatically advance. If your skill is high enough you 
  will be able to write more novel types / genres.
3.The most money can be made from "Romance Novels". You get royalties for 6 weeks once you 
  finish a book.
4.With the "writing" skill maxed out, you can make around 40.000 every week, which is 
  more than 34.800 you get by purchasing all neighborhood businesses and takes less 
  time to achieve.
5.In three game weeks you should have super high-speed writing, 9/10 novels as best-
  sellers and 100.000