To get Rocket Fuel, you will first need to craft an Empty Canister and carry some in your inventory. Then, you will have to find some Crude Oil. Use a bucket to get oil. Make a Refinery to refine the oil into usable rocket fuel. Alternatively, other forge liquids are also usable, such as Buildcraft.
You will need to fill a Fuel Canister in the Refinery, to be able to use the fuel.
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 (currently this is possible only in Creative mode or with aid of other mods), be careful: if there is any fire near it, it can cause an explosion. (Feature added in Galacticraft version 220.127.116.115)