have a look at http://www.isle-of-man.com/manxnotebook/famhist/genealgy/index.htm
however first write down a few known dates - and names; have you birth + marriage certs for grandparents ? - if not can you get then from General Registry in Douglas
does this get you back to 1901 census ? - if not get certs for great grand parents - then use census indices at Manx Museum to find entries in actaul census info - assuming all were on Island that will get you back into parish records but be warmed there is generally a block around 1780 as too many people with the same name - wills often help
here