Build a basic portfolio before demanding a price. This is valuable for several reasons. Editors will want to see samples of your work and for this you will need to build a basic food writing portfolio. You can do this by either starting your own food writing website/blog, or appearing as a guest writer in established magazines, papers, or websites. It's even useful to become a regular contributor to review on food websites to get some experience on writing basic restaurant reviews. All you have to do is-

• Go to a new restaurant.
• Go along with friends, and try and order food from different cuisines. This will add diversity to the kind of food you will be tasting.
• Order something different for everyone and taste from each and every dish. Do remember to jot down your immediate reactions, and take several snapshopts of the food on the table.
• Also, do remember to pay attention to the ambiance and service. Food writers often work with an extensive checklist, which goes much beyond the general impressiveness of the food. 
• Now post this review online and remember to contribute to the comments sections!
• You can also send the reviews to magazines and newspapers, and start your stint as a freelance food writer.