L.M.
The general rule is to start introducing cereal at 6 months. You mix the cereal (usually starting with rice) with formula or breast milk. Then you can gradually add foods starting with fruits or veggies.
IMO you should avoid the juice until at least one year. You child can get fluids from breastmilk, formula or water. When you do introduce juice it should be watered down - at least 50% water.