There are two ways of obtaining fuel.
For this method, you will need to find some Crude Oil. Use a bucket to gather it and place it in a Refinery, which will convert the oil into fuel. You can store up to 24000 units of fuel in the refinery. You can then pump it into a Fuel Loader, Fluid Tank or place an Empty Canister in the output slot.
For this method, use a Water Electrolyzer to gather Hydrogen, which can then be combined with Fragmented Carbon in a Methane Synthesizer to make Methane Gas. This can then be used in a Gas Liquefier to make rocket fuel. Again, this can be pumped into a fuel loader, fluid tank or empty canister.
How To Use Rocket Fuel
In order to launch the Rocket, you must place a Fuel Loader next to a Rocket Launch Pad. Place a filled Fuel Canister into the left hand slot in the Fuel Loader, which will automatically transfer its fuel into the Fuel Loader's internal tank. Give the Fuel Loader some power by inserting a charged Battery or connecting up with Aluminum Wire. If done correctly, the Fuel will start loading into the Spaceship. After the Fuel has fully loaded into the Spaceship, the player can launch. If you are planning to go to the Moon make sure to give the Spaceship plenty of fuel, because you will need enough to come home again: see Pre-flight Checklist.
Warning: highly flammable!
If you have a liquid block of Fuel in your game world, be careful: if there is any fire near it, it can cause an explosion. (Feature added in Galacticraft version 184.108.40.2065)