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.
The customary unit of weight to weigh an orange, for me, would be Ounces. For someone who is accustomed to using the metric system, I would imagine them to use grams, or kilograms.
A scale.
you can't use kilograms to measure a orange, but you can use grams.
to get the correct weight
A spring scale is rather a hit or miss manual weighing device. It should be taken a a guide, rather than an exact weight.
cm or inches
You could use Kilos but I think it looks better if the weight is written as a whole number than part of a whole. eg 130 grams = .13 kilos
Scale
If you mean measure height then it would be the metre if you mean measure weight it would be Kilograms
you use a miltimeter and dissimilar metals
grams* * * * * No. Grams are a measure of mass not weight. Weight is measured in Newtons or, in this case, millinewtons.
Weight and/or mass
to measure weight