If you are on-Island then just visit Manx Museum.
If not then I presume you know all about the IGI (www.familysearch.org) - possibly the material under genealogy on Manxnotebook (see tab bar top of page) might help. Pre 1841 then the parish regs are key - they (or rather the 1911 official transcription) are on mormon microfilms and should be consulted. Then are Memorial inscriptions but these seldom cover many deaths before 1780, wills and decrees (see www.lawsons.ca) are key pre c.1800 but post late 18th C not all poor families will feature. The other key resource is property records (see Manxnotebook - look for example how Wally Corrin used then in Arbory where parish records don't survive. Finally of course join the IoM Fam Hist Soc - look under genealogy on tab bar.