Depending on what position you get the contract normally lasts 6 months, some are a little more, some a little less. This isn't a program like the ICP and the CRP it is a 'real' job so, if you like working on the ship and they like you, you can renew your contract and after the month or two that you have to have off you can go back onto one of the ships and work for another 6 or whatever months. To apply go to http://www.dcljobs.com and find the area you want to work in and then you should be able to find the recruitment agent nearest to you from there with instructions on how to apply. Read the site first and learn as much as you can about DCL, just to make sure you know a bit about the company before you apply. I hope that helps you a bit.
