A career in food writing is incredibly interesting and fun. It is always different and hey, you get to eat your words!
I would say to get started, you should find a subject that you are passionate about and begin developing recipes and headnotes. Pretty soon a book or proposal for a book will take shape.
When developing recipes (I try to develop 4 new recipes a week) make sure that you take exact notes so that you can write them up on you computer later. I have spiral notebooks that I use when I cook, and write down the ingrediants or spices that I use in small increments as I cook. Then always retest your recipes three times to make absolutely certain that the results are the same.
A great book for guidelines for recipe writing is called "Recipes into Type" by Whitman and Simon. Remember that writing recipes is essentially technical writing because you must convey clear instructions for you readers to follow.
Once you know what you want to write, books or magazines for instance, the next step is marketing your work. A great source is called "The Writers Market" and a new edtion comes out every year.
The most important thing to remember is that you must never get discouraged! Every writer has stacks of rejection slips and you just have to find the right publisher that shares your vision.