Well, it's not as simple as not eating meat.
I've been a full-on vegetarian for 3 years now. For many years before that, I ate no red meat, but did occasionally indulge in poultry.
Here are some of the things to consider:
1. Why do you want to be a vegetarian? The answer to this will help inform the answers to some of the following questions.
2. What
type of vegetarian do you want to be? There are many. Lacto-ovo is the most common, by which you eat some animal products, but never animal flesh. There's also dietary vegan (you don't eat any animal products, but don't necessarily avoid all animal products for other uses [eg clothing]). Another less-known variant is "pescetarianism" where you'll eat occasional fish, but no other animal flesh. Which one of these, or other innumerable variations thereof you choose is largely based on the reason why you want to be a vegetarian.
3. What is your budget like? Some meat replacements are expensive. You may need to adjust your answer to number two to fit your budget.
Once you've answsered the above questions, do a lot of research. You'll need to get a minumum of 60 grams of "complete" protein per day. Complete protein provides all of the
essential amino acids. Some sources will claim that Soy provides a complete protein. This is technically false. While it does contain all of the essential amino acids, it falls short on the quantity required. Most people are capable of attaining the full set of amino acids by pairing soy (or another legume) with grain (brown rice, whole wheat, whole corn).
I, personally, eat a lot of products that have soy protein isolate (tofu, in essence), and seitan (wheat gluten). This provides a quality protein source, without an overload of carbs.
If you choose to be lacto-ovo, you can suppliment your daily intake of protein with cheese, milk, and eggs. Just be careful, as those sources tend to be high in saturated fats and cholesterol.