Where do you get 8 pieces of stone to make cobblestones for a furnace in minecraft?

When a block of stone is destroyed with a pickaxe, it drops one block of cobblestone per stone block destroyed; to answer, you will need to craft any tier of pickaxe - wooden, iron, gold or diamond all destroy stone blocks - and find either exposed stone blocks in the world, or dig through dirt blocks until you find stone blocks.

The next thing you'll need is a crafting table (4x4 grid of wooden planks in your inventory crafting section) in order to actually create a furnace. See sources/related links for the crafting recipe.