The above links provide some great information. I would also recommend Mariners School.
The sea service time required is: 90 days in the last three years, either offshore or inland and a total of 360 days of sea time since the age of 15. The hardest part of this is ofcourse documentation. If you don't or have not kept a log book, it is tough to go back. I would encourage all youngsters to keep a log,even if it's a rough log; you never know when you are going to need it.