How can I write an essay on the topic of how my eating habits have changed since I was a teenager?
This is, to a large degree, a personal essay, based on your memories of what you used to eat and awareness of what you currently eat. The difficulty isn't obtaining information as the subject is your own memories (though your parents may be good resources -- parents often remember things kids don't). The problem is how to make it an essay rather than a mere list.
The simplest organizational strategy for this sort of paper is one based on categorization and subordination. In other words, start categorizing food into groups. A good way to group foods might be to follow the US Department of Agriculture templates found at choosemyplate.gov. Divide up a piece of paper, and for each major food group, write down foods in that group you ate in the past and foods in the group you eat now.
Do you eat more or less of certain types of food now than you did when you were a teenager? Why?