Here's the equation you need to know for your experiment:
C6H12O6( glucose or sugar in solution aqueous) + 6 O2(gas) + 6 H2O(liquid)â†’ 6 CO2(gas) + 12 H2O (liquid)
glucose + oxygen â†’ carbon dioxide + water
It sounds like you will be collecting the carbon dioxide from your fermentation sample. If you measure the volume of carbon dioxide produced in your experiment, you can calculate the exact amount of sugar used and the amount of carbon dioxide produced. Here's how to measure the amount of carbon dioxide produced:
1 mole of carbon dioxide = 22.4 liters at standard conditions.
To understand this better, you will need to learn about the ideal gas laws. Here's a good website for this subject:
If you have taken chemistry, this will be familiar, but if you haven't you will need to study this subject and make sure you can explain it in your background research. You will also need to show the calculations you used to convert the volume of carbon dioxide to moles or grams.
Your question about the source of yeast is a good one. Ideally, you would want to use a pure culture of yeast, grown to the same growth stage, and used in the same concentration in each experiment. If this is not possible, (and it probably won't be unless you have access to a microbiology laboratory), then you can buy enough bakers yeast from one lot of one brand (purchased at the same time) and use the same amount of this yeast for each experiment. The idea is to keep all parameters of the experiment identical, except the variable, which is the liquid you are using to grow the yeast.
I don't know what grade you are in, but do let us know if you need help understanding the equations or the gas laws.