A thesis statement should tell the reader your belief about something and demonstrate that you have evidence to support that belief.
Your thesis statement could say that you believe "eating disorders is a worldwide problem." But that's not a very strong opinion. If you were to say, "eating disorders is a worldwide problem that is killing us too early," it would be a stronger statement and we'd be more likely to want to read more. Then you would add what type of problem it is (is it psychological, neurological, chemical?), and what can be done about it.
A good thesis statement needs to present your opinion in a convincing manner, encourage the reader to continue reading for your evidence, and provide a guide for the reader to know where your paper is going (how your argument will be presented).
Do not be discouraged if, once you begin your research, you realize that your original thesis statement was too broad a topic. That is, you realize you've bitten off more than you can chew. You can always narrow the focus of your topic to present your argument more convincingly and fit within your assignment guidelines.
The links below can also help you with your assignment.
One of the things that I would look at in regards to eating disorders is the obsession people have with food. Either they are afraid of the weight they will gain from it so they monitor every bite and caloric count and purge or exercise to rid their body of it. The opposite end of the spectrum is the obsession that one needs more than one should eat. It is not uncommon for someone to drive around looking for which fast place to eat at before he or she intends to go on a diet. When the next day comes he or she falls off the diet and the cycle starts over again.
One of the main reasons that people start putting weight back on after having had gastric bypass surgery is because food goes from an obsession of eating it to trying to figure out what one can eat.
If I were to look at a thesis for eating disorders being such a world wide problem, I would look at the availability of the variety of foods and the individuals psyche that makes them crave and obsess about foods.