Destroy target land. Its controller may search their library for a basic land card, put it onto the battlefield tapped, then shuffle their library. Draw a card.
If the target land is an illegal target by the time Cleansing Wildfire tries to resolve, the spell doesn’t resolve. No player searches for a basic land card and you don’t draw a card. If the target is legal but not destroyed (most likely because it has indestructible), its controller does search for a basic land card and you do draw a card.