I think you will find the main variant of this Manx name is Cretney....

You will need to have some idea of your starting point (parent/grandparent with name and a date details) and then work backwards.
Go to:

http://www.isle-of-man.com/manxnotebook/

click on "search page" and type in your name and search the many Cretney references.

Also go to Brian Lawson's site www.lawsons.ca and search the marriages, wills, and burials indexes.

www.familysearch.org should give you many references to your name also using the search facility selecting the Isle of Man as country of course. You will need to be vigilant in using this site as, while it is a wonderful resource, it is prone to many inaccuracies.

Use the search facility at http://rootsweb.com also as I am sure you will find Cretney refs there also.

Good hunting

Elizabeth