I am having problems with the Database.mdf and would like to use my own database but i don't know what fields i need in the tables, as i can't open Database.mdf

You dont have to know wich tables you have to create. You only need to modify the Connection String in the db.config to point to your sql server and database you want use.

On the First Run, when you open up Yaf. The YAF Installer will automatically Creates all the Tables and the other stuff for you