Running out of fuel is a constant risk within Elite: Dangerous if you do not remain aware of your fuel usage. Fuel is used with every ship action; merely having your modules switched on will use a tiny amount of fuel, with the amount required increasing with thruster use and supercruise travel. A hyperspace jump to another system will typically use a significant percentage of fuel.
You can refuel at stations whenever you dock. You can also equip a Fuel Scoop and collect fuel from skimming past a suitable sun.
Finally, a transfer of fuel from one CMDR to another is possible through the deployment of Fuel Transfer Limpet controllers.
Be sure that you adequately plan your journey and that you have plenty of opportunities to replenish your fuel levels along your route!
However, if you've come to this page because you are currently out of fuel and unable to reach safety, then the Fuel Rats may be the ones to call! These fine pilots travel the galaxy rescuing stranded ships, providing them with the fuel they need to reach safety.
To request assistance from a Fuel Rat, contact them on their official website or alternatively send them a message on Twitter!
"We have fuel. You don't. Any questions?"