Start by learning as much as you can about vitamin C. What does it dissolve in, and what does it not dissolve in? This will help you formulate a plan to get it out of a food source. Then lets see your plan, and we'll give you some more tips.
Learn what a spectrophotometer is, and how it works, and see if there are experiments in your lab textbook, that you can start to adapt to analyze vitamin C.
You're going to have to write up your report on how you came up with a method, and "some guy on a website says that" is for one thing, not a reference you can count on, books and journal articles are. Also, if you just copy the entire procedure from a web page, the teacher will just say you've done no work, and give you a zero.
We do Socratic learning, 'round here. You ask a question, we give you a hint, you ask another question, we say you're right or give you another hint, like that. Some noobs have taken to just dumping a complete answer lately, but they're only cheating themselves and you, in the process.