Many people become vegetarian overnight, while others make the change gradually.
Do what works best for you. Replace the beef in burritos with beans or grilled veggies. Use crumbled tofu instead of ricotta cheese in manicotti and lasagna. Use crumbled veggie burgers instead of ground beef. Many canned-soup flavors that you're probably already used to are vegetarian, like black bean, minestrone, tomato, and vegetable.