If you have studied the subject and know the difference between animal proteins and vegetable proteins and if you know how to cook to provide yourself with all of the protien you need plus all of the vitamins and minerals that you can only get through animal sources. Then you can begin by just doing it. If not, then I strongly urge you study the subject in depth, weigh the pros and cons and learn the differences of proteins of animals compared to proteins of veg. Also learn the proper use of supplements, and most importantly learn how to cook veg and fruits without stripping them dead of anything of nutritional value. Further you are going to have to weight the meaning of "vegetarian" for yourself. And you have to learn what is served at where ever you dine out and how they prepare food. Expect to spend at least (yes at least) 6 months studying the subject and weighing the pros and cons - unless you can spend 8 hours a day studying the subject - I assume you have a job/school/ and that your time is already used up in other things and study of vegetarian life is only something you can do in your "spare time". There are a lot of great books out there. Seriously go to your local library and look - row upon row of shelves cover this subject and the attending nutritional books. Check out a few and read them - if you find one you want you can usually get a copy or contact the publisher for a copy to own. Good luck in your new life style. BTW you will note I did not say consult your doctor before you start a new diet - If you have studied then you already know this, if you have yet to study you will find this disclaimer to be very popular.

Slowly

stop eating meat.

Buy vegetables.

The best way to begin is not begin! We are omnivores and have evolved to be so! All herbivores are prey (with a few exceptions) in the wild and carnivores are predators. We are successful enough to benefit from both, so why go against human nature? Of course, if you want to be a vegetarian, that's your choice and no-one should put you down for it!

Go to peta2.com. That site convinced me to go veg. It will tell you how to start off, what to eat and other helpful things.

Know your personal reason for wanting that:i.e. not peer pressure, popularity, because of a fad, or you just saw some kind of meat butchering film, or meat processing film. A real legit personal desire and reason. With that shared, obtain Justified information from a reputable source, FDA, Vegeterian Society, etc. and know the nutritional values that need to be replaced for what you'll be taking away. Proteins, carbs, fats, vitamins (especially the B vitamins). check out books, learn from others.

I became a vegetarian in one day and made a firm point of being one.My father,before I was born quite smoking in one day and never went back.It takes willpower to do so. I would first cut out all fried meats,they are the worst.Then cut out beef,then chicken ,then fish.Eat lots of salads and pastas.