From Pokémon Emerald onward, if either parent of a baby Pichu holds a Light Ball when the Egg is produced, the Pichu will know Volt Tackle when it hatches. However, Volt Tackle is not an Egg move, and cannot be passed down by simply breeding with a parent that knows the move. The only way for Pichu to hatch knowing Volt Tackle is for one of the parents to hold a Light Ball.
So basically, just because you have a Light Ball doesn't mean the Pichu will get Volt Tackle. You need 2 Pikachus that are opposite in gender to breed. One of the parents must hold the Light Ball in order to get an offspring Pichu that knows Volt Tackle.
In order to obtain the Light Ball, you must catch wild Pikachu. They have a 5% chance of holding a Light Ball (XY).
Source (How to get Volt Tackle on Pichu)
Source (How to get the Light Ball)
Also, this question has been asked before here .