The best way to educate yourself on the benefits of a healthy diet which would be best for you is by visiting a nutritionist or other certified professional in the area. A certified nutritionist will have the proper licensing to determine which types of tests you should undergo prior to changing your diet. What may be healthy and good for some may create an allergic reaction on you, so be careful.
As far as options, there is a current tendency to promote what have become known as "super foods". These are foods that are allegedly full of antioxidants that may help battle chronic diseases and even cancer. Among these superfoods we have the "leafy greens" and "dark green" vegetables such as spinach, kale, and arugula. These are thought to be the best choice for making salads.
The other green giants are avocados. The properties of folic acid contained in avocados, plus its cholesterol-reducing oils, make of it a good source of what is known as "good fat".
Salmon, another super-food, has been associated with healthier levels of cholesterol and a healthy weight, for it is a nutritious and filling food that can be prepared in many ways. Berries such as acai, cherry, and goji berries are now considered to be super antioxidants. Nuts, combined with the proper berries, make for a wonderful snack.
As a healthy eater myself, I tend to buy shiritake noodles (tofu-based) which are a total of 40 calories per entire package, less than 45 mg of sodium, no carbohydrates, and 19G of protein. I mix that with a combination of unsalted butter and low sodium sour cream, add vegetables galore and it is basically like eating Alfredo without the fat, junk, and calories.
A salad is not always a good option, since many chef and ham/cheese salads are loaded with pre-packaged meats that contain loads of bloating salt and preservatives. A kale or spinach-based salad with avocadoes, ahi tuna (or raw salmon like sashimi), almonds, and berries make such a tasty salad that it may not even need dressing.
Always watch out for salt content, sugar content, and the additives of food. Once you move away from that, you will find yourself finding out how natural and organic food is the way to go. Move away from fried, pre-packaged, or frozen and you will be on your way to a much healthier life.
A healthy lunch should have some carbohydrate for energy and protein for strength. It should also have colorful vegetables to provide key nutrients (vitamins, minerals, etc). Some healthy lunch components include: pasta, scrambled eggs, salads, soups, stir-fry, and steamed / boiled vegetables. A healthy portion of good fats should not be left out for fear of gaining weight.