The first consideration is to stop the flow of oil. You can do this by turning off a valve, plugging a leak, stopping a pump etc. You must then stop the leaked fuel getting into drains, finding its way underground, getting into water courses etc. This can often be achieved by making small dams with sand or soil and by placing absorbents on or in the path of the leaking oil.
Once the flow has been stemmed you need to contact an OFTEC registered tank engineer immediately who can attend and pump out the remainder of the oil to prevent any further leaks. They will also be able to advise on the process of replacing the damaged tank.
If you have a small leak on your plastic or sometimes steel tank you can quite often do a "get you out of trouble" repair by rubbing a bar of soap over the split/hole. This fills the gap with soap and may give you a little more time to cope with the problem. This is not by any means a permanent fix but may stem the flow of oil until a tank engineer can get to you.