On this page of the guide to Void Bastards, we list all the main methods of obtaining fuel. It is important to know how to get fuel - without it, you won't be able to travel through the nebula.
The first and most obvious way is to find fuel as loot. Every time when you explore an enemy ship, go to the FTL room (an example presented in the picture).
In FTL rooms, you should find at least a few fuel canisters. Each one of them gives you 1 fuel unit which allows you to perform one move on the nebula map.
Remember, some more "advanced" ships store their fuel in TANKS room instead of FTL (see the picture above). Also, fuel canisters stored in these rooms can be heavily guarded.
The second way is to buy fuel on one of the Krell-Mart ships. However, spending credits (Merits) on fuel should be the last resort. Do this only if your fuel supplies are running low (1-2).
The third way is to find fuel on the nebula map. Look for Fuel Cells. Once you reach such a field, you will get a random amount of fuel.
The fourth way is to order fuel in a FILE room. These large and round rooms appear only in specific ships. There, you will find a device with three levers - choose the one for fuel deliveries. Orders are free.
After a few jumps/turns on the nebula map, the ship with ordered fuel will arrive.
It is worth adding that you will be able to save fuel in the later parts of the game. You can buy the Fuel Scavenger blueprint at the Workbench. It provides a 25 percent chance that moving to another location won't consume fuel. To unlock this blueprint, you need Lates Seal, which can be made from crafting materials or found on one of the visited enemy ships.