That sounds like a fantastic project! Make sure you wear safety glasses because it would really hurt to get vinegar in your eye. I think three antacids is a good number to test.

I can think of two ways to graph the results. The easiest is to simply time how long it takes for each antacid to neutralize the vinegar solution and then make a bar graph showing the time for each one. If it doesn't change color, it means you used too much vinegar/not enough antacid. Even if you want to do the more complicated experiment, I would do this one first to make sure everything is working properly.

If you want to do something more complicated, you could try to actually estimate the pH based on the color of the cabbage juice. Then you could write down how long it takes to change to each pH. This might be difficult because the color change will be gradual.To do the second experiment, you need to know which color goes with which pH.It looks like:

These website have pictures of what red cabbage juice looks like at different pH levels so they might help in determining this.

Then you can graph pH versus time for each antacid. Household vinegar should start around a pH of 2.5 to 3.5 and hopefully if you use enough antacid, it will be neutralized to a pH of about 7.

As for the report, it should include something similar to the following sections:
Introduction,
Question/Hypothesis,
Materials and Methods,
Data and Results,
and Conclusion.

The introduction should explain essentially what you explained in youre-mail. You should explain what antacids are, what they do, and why you want to test them.

In the hypothesis section, you should include your question: Which antacid neutralizes acid fastest? and a hypothesis like "I think Tums will work the fastest" or "I think it will take 10 minutes for the antacids to neutralize acid." You could also explain why you are making that hypothesis.

In the Materials and Methods section you should include a list of all of the materials you used, and also a list of the steps you performed for the experiment. Explain how you extracted the cabbage juice.Explain how you made the vinegar solutions. How much vinegar did you use? How much antacid did you use? etc.

In the Data and Results section, you should have your graph of your results and also any observations about the experiment.

In the conclusion, discuss what the results were and how they relate to your hypothesis. What did you learn from doing the experiment? Was there anything unexpected?

If you have more questions, feel free to e-mail me again. I am happy to help as much as I can.