Some properties of hydrocarbons depend on the size of their molecules: boiling point, viscosity and flammability. As the molecules get larger, the boiling point increases, the hydrocarbon becomes more viscous (thicker, so it flows less easily), and it becomes less flammable (harder to ignite). Small molecules have a low boiling point, flow easily and ignite easily. These properties decide how each hydrocarbon is used as a fuel. For example, a small-molecule gas is easy to light and burn, while a large-molecule fuel is thick and needs more effort to ignite.
Burning a fuel is combustion, and it releases energy. During combustion the carbon and hydrogen in the hydrocarbon are oxidised. With plenty of oxygen this is complete combustion, and the products are carbon dioxide and water.
You need to write balanced equations for complete combustion. Methane: CH4 + 2O2 → CO2 + 2H2O. Pentane: C5H12 + 8O2 → 5CO2 + 6H2O. To balance, put one CO2 for every carbon atom, then one H2O for every two hydrogen atoms, and finally count the oxygen atoms on the right to find the number of O2 molecules, which is that total divided by two.