Its a good sign that you are asking this question now and not several weeks into your new lifestyle. Focus on getting your nutrients from food, then supplement with a good, vegetarian multivitamin.
There are a few things you can do. First, go to this web site and find out what your daily requirements are for age, gender, etc.
Nutrition facts, calories in food, labels, nutritional information and analysis – NutritionData.com
Then, write down everything you are eating now and use the search bar on top of that site to find out the nutritional value of what you are eating now (be sure to include quantity info). Check this against your daily requirements you found above so you'll know where you stand going into the process (and learn how much sat fat and how little fiber there is in a meat-based diet).
Now, follow the steps here: http://www.vegetariantimes.com/2007/pdf/vegetarian_starter_kit.pdf (broken link)
As you reduce the amount of meat in your diet to eventually reach zero, keep writing down what you ate and measure it the same way. After a few weeks, you won't need to look them up any more because you'll know how protein rich broccoli and beans are, etc.
If you want to be even more prepared, see if your library of bookstore has a copy of the book
Becoming Vegetarian. Its an excellent guide to meal planning written by two dieticians.
The best part of being well informed is that you'll be able to explain it al to your parents and make them confident that you are making a mature, wise decision - which it is!