In actual fact it is whatever mommy Robin and Daddy Robin eat but regurgitated. So we are talking bugs here as Robins mainly feed on small invertebrates, which they find in the soil. They will hunt through the leaf litter and under bushes, where the ground is sheltered from frosts. Did you know that Robins have very large eyes (proportionally speaking), which allows them to see well in these dark places.
Robins will also eat fruit such as bits of strawberry, blueberry, cherry, raspberry, and other fruits. I know from first-hand experience that they are partial to a custard cream.
If you find an uninjured baby bird you will need to figure out if it actually is an orphaned bird. One sure way is to conclude if it is a nestling or a fledgling. The majority of young birds that are found are fledglings that are still a bit rubbish at the whole flying thing.
Firstly, you need to allow the young bird to perch on your finger. If it grips your finger, you've got a fledging, if not it's a nestling. For fledglings the best course of action is to pop it in a nearby tree or shrub and let it get on with things. It'll be fine. For nestlings, you need to locate the nest. It'll probably be close by and well hidden. Pop the bird back in the nest and don't worry about leaving a scent on the baby Robin, the parents identify their young by sight and sound.
You should not hand feed baby birds but if you feel that you must, you can buy hand-feeding mixtures (available at pet shops for hand-rearing baby parrots and cockatiels) or mush up any of the food mentioned above.
I'll leave you with some sage advice 'never force water into any bird'.