Most lambs feed on grass, clover, forbs, and pasture plants. Forbs are very nutritious and more preferred than grass. When the lambs are young, they feed on their mother's milk for sometime before they are introduced to solid foods.
2 Additional Answers
Sheep and lambs are herbivores and spend most of their time grazing. They like to eat all types of grasses and clovers. On a farm, they might eat feed or other grains. You can find more information here: http://www.sheep101.info/eating.html
Lambs are what baby sheep are called. They only eat their mother's milk, because this is all the nourishment they need at this time in their lives.