To build a fire you will need to obtain wood. You can find wood piles scattered around the land, or you can use an Axe to chop down a dead tree. With Logs in your inventory, click on a Fire Pit and select "Use". The logs in your inventory will be replaced with a fire.
You will earn 1/10th your level in experience everytime you light a log in a firepit, so if you are level 20 you will earn 2 experience points per fire.
Many of the shops in Queville sell "Matches", and some of the enemies you fight will drop them. Matches come three to a stack, each one can light 1 fire. You will earn 1/4th your level for every match that you light.
To use a fire, simply click it in your inventory and select "Drop". You cannot drop a fire on a tile that already has a fire. You can use this fire to cook fish, or smith on a furnace. You can also "Use" the fire to light logs in your inventory for 1 experience point.
The Lava Mountain is a dangerous section of Queville, where rivers flow with Lava. You will need to be wearing Lava Boots or Travel Boots to walk on the Lava. Here you will find "Burning Trees" that you can chop down with level 40 Arson.
At the heart of the Lava Mountain is the Lava Pit ... inside this pit is where Lava Ore can be mined; you can enter this pit with level 60 Arson.