Don't eat gluten. Simple as that!
When you're first diagnosed with Coeliac Disease, "no gluten" sounds easy: no bread and cake. Then the list starts to grow.
The toxic part of the gluten molecule is prolamin. That's gliadin in wheat, secalin in rye, horedin in barley and evedin in oats. The gluten found in corn and rice doesn't contain prolamin but that still leaves plenty of things that you're about to kiss goodbye to.
No cereals with the above grains; nothing with added malt extract; pasta; biscuits and crackers; anything with gluten-based stabilisers (cold meats, tinned vegetables and fruits, sandwich spreads); glazed or self-baking meat, etc; egg substitutes; some frozen or flavoured yogurts and milk drinks; most salad dressings and sauces; soups; diary substitutes; herbal teas, cocoa drinks and instant coffee; beer and many shorts.
When you've managed to snap your mouth shut and blinked yourself out of your amazement, you realise why depression is not only a symptom of Coeliac Disease but often a consequence of diagnosis. After a few days -- when you've started the mourning process and accepted that there's no way out of this -- you can start to look for alternatives.
If you've always eaten a health diet with few processed foods, you'll find a gluten-free diet easier than if you're a packaged food junkie. A balanced, gluten-free main meal really isn't too difficult to manage; just stick with what the home economics teacher lectured you about at school. Breakfast can be a full English or just a bowl of plain cornflakes. Lunch, in my experience, is the biggest problem and is probably even worse if you have to eat at a works canteen. Sandwiches are off the menu unless you actually like the hideous gluten-free stuff. There are gluten-free crackers and biscuits, though, and you can always bake or buy gluten-free cake. There are possibilities and alternatives, you just have to find them.
Eventually, you'll stop focusing on what you can't have and start lining up stuff that you can eat. Shopping can be a bit of a shock for a few weeks but things settle down and you get in to a new routine. Then, somewhere along the way, you realise that you're actually surviving on a gluten-free diet.