To be healthy at work first of all start your day with a good break fast.It helps you as a fuel of body and leaves you energetic and active throughout the day.
*Along with you carry some fruits with you which you can take in mid morning and mid evening meals or whenever you feel hungry.
* Keep some healthy snacks at your desk such as roasted nuts,roasted chana,roasted murmura,multigrain biscuits/fiber biscuits.So that whenever you feel hungry you can go for these healthy foods rather then havink the junk food in canteen or outside.
Try to avoid coffee.If you can have green tea/hearbal tea over coffee will help.