Minecraft wiki entry for Smelting, associated text: If you are doing 16, 19, or 22 processes, start by burning 2 coal or charcoal.
If you are doing 17, 20, or 23 processes, start by burning 1 coal or charcoal.
If none of the above are true, just burn slabs, wood planks, or banners. You can also start by burning a blaze rod.