What qualifies as a business expense?

I am a professional organizer. I have a home office where I do all of my billing, networking, paperwork, writing, etc. I am wondering what I can "write off" as a business expense. Some things are obvious (I track my miles driving to and from clients). But what about the food I bring each work day to eat for lunch? When I grocery shop, I pick up items to eat when I work but it's all on one receipt. I'm curious to know what other items are a tax deduction. Clothes I wear only for work? Equipment I buy for work?