I only eat one yougourt a day and thats all ill eat is that ok?

A yogurt is about 90 calories- no, that's not all you should eat in a day. Try to eat some protein, whole grains, veggies, and fruit for a balanced diet. The most sedentary person needs at least 1000 calories a day. If you're active, you'll need more than that. You say you don't have a big appetite. What about your energy level? You need to eat- seriously- or you'll end up in the hospital. Try to at least have a bowl of cereal and some liquid meal drinks like Ensure.