Every player has a 2x2 crafting grid in their inventory, but a Workbench can be used to craft items requiring a 3x3 crafting grid.
The most basic crafting recipe is Wooden Planks. First you need to find a tree, walk up to its trunk and hold down the Attack button (Default is Left Click). Cracks will start to appear on the Log you are attacking and once its "destroyed" the Log will drop onto the ground and start to spin (Unless your close enough for the Log to be automaticaly picked up). Walk up to the spinning Log to pick it up. Now that you have a Log you can craft Wooden Planks. Open your inventory (Default is the E key) and place the Log into the crafting grid. Left Click on the 4 Wooden Planks and then Left Click on a slot in your inventory.
You can create a Workbench by placing a Wooden Plank in each slot in the crafting grid of your inventory. To use a Workbench, place it on the ground (Default is Right Click) and then activate it (Default is Right Click).
You can find a complete list of crafting recipes Here.
You use a Furnace to smelt items, smelting allows you to cook food and smelt ores. To use a Furnace, place it on the ground (Default is Right Click) and then activate it (Default is Right Click). Food which is cooked fills your hunger bar up more than un-cooked food, it also makes food like Raw Chicken safe to eat (Raw Chicken can poison you if eaten). Ores like Iron Ore need to be smelted to be used in crafting recipes. To smelt an item you need to place it in the top slot with a burnable material in the bottom slot. Coal or Charcoal are the best forms of burnable materials.
You can find a complete list of items which can be smelted Here.