Press E. It should leave the furnace GUI.
To access a furnace in minecraft, simply click it!!!! Its that easy!!
You can't...
Smelt Cobblestone in your furnace.
yes
You automatically get 1
Burn cactus in a furnace.
Burn cactus in a furnace.
You put cobblestone in a furnace with something to cook it with
You can't.
You put sand in a furnace
Shift while you place it.
There is no steel in minecraft. You're referring to iron, and you need to smelt iron ore in a furnace.