Tomb Raider: Legend

A fuel tank is a grey tank and looks much like a gas canister rather than a fuel tank. The first place you will encounter fuel tanks are during level 2. When you reach the courtyard at the very first part of the level. Look to your right upon entering, there's one next to some boxes. If you kick the fuel tank up (away from the truck) a bit and then deal with the enemies. Once You've killed the first wave 2 guys will run out the doors right by the fuel tank, you will defiantly be able to kill one, maybe both. To explode a fuel tank look towards it while pressing and you will see a prompt above it, press when ready to detonate it. There's another one in the same courtyard if you fail. If you don't get it this time there's plenty more chances during this level and level 5.