Sponges cannot be crafted in Minecraft
Obtain sponges from Ocean Monuments
Find a Sponge Room inside the monument
Break the sponge blocks to collect sponges
Defeat the Elder Guardian for a chance to get a wet sponge
Dry wet sponges in a furnace, blast furnace, or by placing them in the Nether or in the Overworld with a fuel source in a furnace