Get Pichu in Pokémon Emerald

Pichu is not hard to get in Pokémon Emerald. However, you do need to be able to use the Safari Zone.

Steps

Go to the Safari Zone, north of Route 121.
Catch a Pikachu male and a Pikachu female in the patch of grass west of the first patch you see.
Put both Pikachu in the Daycare west of Mauville City.
Come back later and talk to the daycare man to receive an egg.
When the egg hatches, it will be a Pichu!

When hatching eggs, put a Pokémon in your party with the Flame Body or Magma Armor Ability and ride your Bike back and forth from the beach to Vendanturf Town.
If you want your Pichu to know Volt Tackle, catch one of its parents while it holds the light ball.
If you don't want too many Pichu, take your Pikachus out after you have enough eggs or you'll end up with 60 Pichu that you don't know what to do with.