I found a paper online that I think explains what you're trying to do:http://www.insinet.net/rjabs/2007/367-369.pdf
The way it's written, it may be too advanced for you, but I wanted you to know where my info is coming from and it's something you could show a teacher if you need more help.
Have you covered moles in chemistry class yet? That's a concept you need to understand for this project. A mole is 6.02 x 10^23 molecules. Depending on the size of the molecule, a mole of different compounds will have a different mass in grams. If I remember my chemistry correctly, a molecule's mass in g/mol=atomic mass in Daltons, which you get by adding up the atomic weight of each individual atom.
For example, water=2 hydrogen + 1 oxygen. H=1 dalton, O=16 daltons, so water=1+1+16=18 daltons, or 18 g/mol. That is, if you have 18 grams of water, you'd have a mole--6.02 x 10^23 individual water molecules. (You can get these numbers from any periodic table.)
From this paper, it looks like every molecule of vitamin C neutralizes one molecule of iodine in the starch solution. So, what you need to do is figure out how many molecules, or moles, of iodine are in your solution. That will be the same as the number of vitamin C molecules it took to neutralize it. The molecular weight of vitamin C is 176.123 g/mol (from the paper), so you can use that number to convert from moles to grams. Then, you can use your drop # to figure out how many grams in a ml of juice.
Hope this helps!