How do you get rock climb Pokemon soul silver?

You can't actually get Rock Climb in Pokemon SoulSilver. This move was introduced in Generation IV games (Diamond, Pearl, and Platinum). SoulSilver is a Generation II game (Gold, Silver, and Crystal) so it doesn't have access to that move.

If you're looking for a move that lets you scale mountains in SoulSilver, the closest thing is Strength. You can learn that from the Move Tutor in Goldenrod City. However, it's not a true "Rock Climb" equivalent since it only moves boulders, not characters.