How old are you, what is your current heights and weight and do you need to lose weight?
Just as general tips on healthier eating, get some veggies or salad into your lunchtime meal. Look at other proteins to have, as well as tuna.
Snacks are ok, but watch the calories and don't let them become a habit; you don't need to snack if your meals are filling. Just because fruit and nuts are basically healthy doesn't mean you should eat too many.
Again, vary your dinner and make it the right proportions of protein, complex carbs and veg. Play around with herbs and spices and learn to cook your own sauces, so much better than out of a jar.