There are two basic ways to clean most tanks. One is to do a flushing process and the other one is to actually open it up and clean it. If you can see rust in it, you are going to need to run some good filtration on it even after cleaning or you will need to put a sealer on it.
To flush a tank, basically you pump the fuel at a very high flow rate out of the tank into a series of decreasing size filters and then feed it back into the tank. You need to agitate the tank so a high flowrate is necessary. This is often called fuel polishing.
If there is a large enough access on the tank to allow it and a good way to drain the tank, the preferable way is to pressure wash the inside using a detergent.