My doctor said i should eat a lot of vegetable, small amount of starch and drink lots of water. how can i maintain my weight and not lose on this diet.

Protein? Oils/fats?: If a person is eating just vegetables (like those leafy ones), and a little bit of starch, she is probably trying to lose weight. To be on a balanced vegan diet with low carbs (little starch), she'll need protein such as from tofu or nuts, and probably need some "healthy" fats such as olive oil or nut-butters. She can ask the doctor about the best sources of protein and fats, for her situation.