Is the lemonade diet a good start to becoming a vegetarian?

No: Depending on how much meat you are used to eating would depend on how fast to eliminate it from your diet. If you eat meat with every meal, try having only 1 meal a day contain meat, then you can stretch it to just meat on weekends until you are meat free. Try some meat substitutes (like smart bacon or chik'n patties) and find some new vegetarian favorites, like veggie lasagna, or salads or quinoa.