you need to have the explore kit then you need to dig in the under ground.
The museum in Oreburgh city has a guy behind the counter that will make you a Pokemon from a fossil.
First you have to excavate a Skull Fossil in the Underground. Transfer the fossil to your bag, and take it to the mining museum in Oreburgh City. Talk to the man behind the counter, and he will turn the fossil into Cranidos.
you get it from a fossil
What's a skull fossil
Cranidos
Shieldon.
Defienetly the skull fossil
You cannot get Skull Fossil or Armor Fossil in Pokemon HeartGold.
You cannot get a Skull Fossil (Cranidos) in Pokemon HeartGold.
regirock
You cannot get Cranidos in Pokemon Pearl unless you trade to get the Skull Fossil from a friend. You can only find a Skull Fossil in Diamond.
cranidos comes from the skull fossilwhiteless