Step . 1
Make sure those two are compatible for breeding. One of either Pikachu or Raichu need to be female and male. If they're not get the female one out of those two and breed with a male from the Field and Fairy Egg Group or use a Ditto.
Hand them over to the Dayare located in Route 34. If you want the Pichu to inherit some desirable Stats see here for more detail on Breeding.
Bike or Walk/Run around until the Daycare man comes forward or calls you via the Pokegear.
Recieve the Egg from the man (make sure to have a slot free !) and Bike/ Walk/Run. To make Egg-Hatching Faster have a Pokemon with Flame Body or Magma Armour.
Hatch the Egg and enjoy your Pichu :)
Egg Moves if you want to breed them.