What do you use to measure the weight of an orange?

You will need a set of kitchen scales. These can be bought fairly cheaply from kitchenware stores. Otherwise, ask a friend or neighbour who has a set if you can use theirs. Some departments at your school will also have scales you should be allowed to use.

Or you could take your orange to a local fruit store and ask the staff if you can weigh it on their scales. Fruit stores and greengrocers usually have scales available for customers to use, but you will need to show your orange to the staff before you go in, so that they know it's an orange you've already paid for somewhere.