That depends on its age.
They should be eating for themselves at about 3 weeks old and should be eating seed then - soaked for a few hours in water to soften it is best. Suitable seeds - canary, millet, canola (rape), niger, hemp - anything reasonably small.
When they hatch they would be fed either insects and/or green, unripe seeds by their parents. To hand-rear one, you could feed either a commercial canary egg-biscuit (rearing) food, or soaked complete dog ration along with some soaked or cooked small seeds.
Change the diet from one to the other as the bird ages.