Our pediatrician, who is great, told us early on: Once they are ready for liquids other than formula or breast milk, ONLY, and in this order:
Water
Milk
Calcium-fortified orange juice (usually watered down a bit)
I quote the pediatrician: "Apple juice has no nutitional value. Lots of parents give it because kids love it. It's worthless, empty calories."
Orange juice has C, potassium and calcium and vitamin D if you buy the fortified type.
But she emphasized that water is the first and best on the list.
As others noted -- check with your pediatrician about the age to start liquids other than formula/breastmilk.
My daughter is now 11 and still drinks mostly milk, water and OJ, along with only one brand of juice box on occasion--an organic, no-sugar, no-colors brand that she really likes. But she never got apple juice or other juices. I think that giving kids juice, even watered, when they're really young only helps them develop a taste for the sweetness of it. She doesn't like sweet drinks and I think the reason is the lack of lots of juices when she was younger. If only she had never discovered brownies....!