Fire can be used in 2 ways, either complete destruction, or being actually useful.
It's rather simple to use Fire for destruction, simply grab a Flint, Flint and Steel, or Cloth Torch and click on anything that can be set on fire! Keep in mind you may need to click multiple times depending on your lighter's effectiveness. Fire can spread very quickly and can become an extreme disaster to your tribe, crops or village! So be sure to not set anything on fire close to something else! Your precious Fire can also be extinguished by those pesky ROBLOXians with Tin Buckets, Wooden Buckets or Clay Jugs!
You can combine Fire with a Cart and create a moving vehicle that can spread fire, but you're going to have to make sure a ROBLOXian in the back can extinguish the fire that gets out of hand, so as to not destroy the cart.
Useful Fire can be used in cooking most Uncooked items, getting rid of some dusty buildings and burning ores. Your food can also get burnt, so watch out! Fire can also be used to drag-burn, wich is dragging a burning object using a glitch and using it to spread fire to a far away location, this however is NOT ALLOWED and can result in a ban from the game.