As with all proffesions. you get to skill level 150, then, providing you are lvl 20+, you go to a proffesion specific trainer(in this case cooking trainer). they will allow you to learn up to 225. then, providing you are over level 35, you can train again. sometimes it is possible to buy a book to increase your maximum skill level.  You can search through Thott for the book, and where to find it, or look in the AH.