You need to evolve it to slowbro first. Then, get an item called "King's Rock", attach it to slowbro, then you trade it with someone else. Eventually after the trade, the slowbro will evolve into slowking.
yep, pisces is right. Train slowpoke so it evolves into slowbro. Then give it the item king rock and trade it. Don't try to evolve it using evolution stones. It doesn't have any effects.