When working out your budget you need to consider all the things you have to pay for other than your debts.
This includes rent, food, cleaning, electricity, transport, phone contracts, school fees etc.
You can always add up what you spent on these things last month to give you an idea.
Also try to add in the small amounts that you should be saving each month that add up to cover big annual costs (eg. A car service or school clothes in Jan),