The most important part of writing a budget is sticking to it. If you write down a list of spending goals, but do not follow it, you may run out of money before something important needs to be paid. For example, if you spent the $30 your parents gave you for the zoo trip before you go, you will not have enough money for the ticket to get in.
Make sure you keep track of the money you are spending by following the budget plan you made. This will make reaching your goals much easier in the future.