The most important aspect of diet is balance. Fruit, salads and vegetables can be eaten in abundance whilst fish, meat, rice, pasta, dairy products, bread etc. should be eaten more sparingly. If this regime is maintained than the occasional treat will do no harm at all.
Cooking destroys vitamins and minerals, so eat as much raw food as possible. Vegetables are better steamed than boiled and grilling is preferred to frying. Where possible, avoid aluminium pots and pans.