If your baby is choking and has not coughed up the food themself you may need to commence CPR.
To do CPR on a baby remember to use not quite as much force as you do with an adult. All new parents should read up on basic child first aid and should keep a book handy.
If your child is choking:

Give your baby a couple of medium hard pats on the back. See if you can easily disodge the food with your finger but do not try too much as you may …

Many babies will seem to to choke when first starting to eat solids. This is usually just a gag reflex at a new taste or, if some food has got stuck, they will often manage to dislodge it themself by gagging. In some cases you will need to help them out with a firm pat on the back or in rare cases CPR.
You can usually tell straight away if your baby is choking:
 

They will go quiet or make a strangled noise.
They will often have their mouth open and tongue out …