How can I improve my diet to make it more healthy?

Healthy diets are also about subtracting stuff. It’s almost always good to add fruits, vegetables and salads.

Cut all white sugar from your diet. Eat chicken breast, fish, lean meat and balance it with vegetables and carbs.

Minimize processed foods. In other words, no granola bars, just oatmeal. No canned vegetables, just fresh ones. No potato chips, just potatoes.

Eat lots of fresh green and yellow vegetables.
Eat several servings a day of fresh fruit.
Eat whole grains such as oatmeal and whole wheat bread.
Eat less salt, sugar, and fatty foods
Do not drink sodas, only water.
Make sure you get enough sleep
Walk at least one mile every day.

I second what @jca said: eliminate processed foods.

See this website for many pages of useful diet information

Be a mankind person and do not over feed. Make exercise every called a day