Welcome to Science Buddies! I think you are doing this really great project, and I’m glad the kit was helpful for you. http://www.sciencebuddies.org/science-f ... p044.shtml
It seems that you primary question is regarding the error in your measurements. The first consideration is regarding the accuracy or precision of the buret. According to this website, which includes a good discussion on this topic, the accuracy of a 50 ml buret or pipette is .05 or .1 ml. http://dwb.unl.edu/Teacher/NSF/C14/C14L ... p01017.htm
The standard error is an estimate of the standard deviation for a group of sample results. Using the triplicate results you obtain from each sample, you can calculate the standard error of your measurements using the following online calculator. The Wikipedia article includes a good discussion of standard error. http://www.miniwebtool.com/standard-error-calculator/http://en.wikipedia.org/wiki/Standard_error
So you should go ahead and do the experiment and measure the results for each sample in triplicate as accurately as possible. After you have all of your results, you can calculate the standard error. I hope this answers your question. Please let us know if you have a question about actually calculating the concentration of vitamin C from the titrated results or if you need additional explanation.