I am breastfeeding and now im very thin, what should i do to gain weight?

Likely nothing: Unless you are severely underweight and malnourished (unlikely, if you are successfully breast feeding), then you probably don't need to gain weight. If you think you might be underweight, see your doctor. But if you are thin, but you eat a good, varied and healthy diet and you keep yourself well hydrated, then stop worrying about it.