In Minecraft, ovens are used for casting many useful items. If you're not sure how to create or activate one, do the following.
Steps
Part 1 of 3: Making the oven

Step 1. Take a pickaxe
It doesn't matter what it's made of.

Step 2. Mine cobblestone
You do this by clicking the left mouse button while looking at stone blocks. You need 8 boulders.

Step 3. Make sure you have a workbench, if you don't already have one

Step 4. Place the 8 cobblestones in the workbench
Arrange them around the edges, leaving the center free. The cobblestones will be transformed into a furnace.

Step 5. Place the oven in the correct spot
You do this by right-clicking on the oven. This will open an interface. This is the furnace interface. You can use this to mold anything you want to make, such as metal rods, wood, and clay.

Step 6. Look in just outside forges in villages
You can then take one with you if you wish (just left click).
It is possible to find ovens that occur naturally in Minecraft
Part 2 of 3: Adding fuel

Step 1. Add wood, coal or charcoal, buckets of lava, blaze rods, or anything that can burn
Everything made of wood is suitable for the oven.

Step 2. Place fuel in the bottom slot of the oven
The item you want to melt goes into the top slot. The right lock is for the object that has been melted down.
Part 3 of 3: Making an oven minecart

Step 1. Make a Furnace Mine Cart
A furnace minecart is a minecart that starts to move on its own when coal is added. Place a minecart in the crafting grid and place an oven on top. Placement is not essential as long as the minecart is under the furnace. The two items will transform into a furnace minecart together. Shift-click or drag to move it to your inventory.

Step 2. Place the furnace minecart on rails to use it
Add coal to it and it will start to move slowly.