To be honest, you should really have though about that before they hatched. However, grain stores and the better pet shops sell broken mixed grain especially for chicks. There is also a ground mash available just for chicks. You can also buy maggots as an occasional treat, which they'll love. Some green vegies/weeds to pick on and renewed daily should see them grow up just fine. Ofcourse, oyster grit should always be available. You also need to provide them with a varied assortment of small grit/stones - for this, coarse riversand used for plastering over concrete is ideal and nearly free. This aids the gizzard with grinding up the seeds, without which they can not get enough nourishment out of the seed before it passes through their digestive system.
They really need a heat lamp over them until they have a full coat of feathers, the time for this varies according to the breed. Lift the lamp higher as the feathers develop and after two weeks or so turn the lamp on only at night. You'll soon see when they are too hot/cold. Huddling together means they are too cold and separate in the corners panting means they are too hot.
Hope this helps.