If you are a newcomer to online family history research in Cornwall, then your first port of call should be to the Cornish pages of GENUKI (xhttp://www.genuki.org.uk/big/eng/Cornwall/). Not only is there a lot of information about the county in general terms, but each parish has its own pages. These pages also contain useful information, but perhaps more importantly, they all contain many links to other Cornish web sites. In particular to the Census Project and the OPC scheme.
The OPC (Online Parish Clerk) scheme was started in Cornwall about 5 years ago and has since spread to 8 or 9 other English counties. It could be called the “adopt-a-parish” scheme, but OPC sounded more interesting. Public spirited people step forward and adopt a parish or parishes. Their aim is to provide information and advice to anyone who contacts them. Some of them have their own web sites and some of them contain transcribed data, things such as parish registers. Others have their data hosted on other web sites and the scheme has its own web site (http://www.cornwall-opc.org/) which contains details of Cornish parishes and of the OPC themselves.
The OPC scheme is about to launch itself into a new project – C-PROP. This is the Cornish Parish Register Online Project and aims to get all 5 million Cornish parish register entries online free-to-view in a searchable database.
The Cornwall Online Census Project (COCP) is a sort of semi-detached part of the Free Census project. The COCP is busy transcribing ALL the Cornish 19th century census returns and placing them online. They are available on the Free Census OLDB (http://www.freecen.org.uk/) and on the COCP web site (http://freepages.genealogy.rootsweb.com/~kayhin/ukocp.html). The whole of the 1841 & 1891 are online plus most of the 51 & 61. The 1871 & 81 are also appearing.
There are of course, lots of Cornish web sites you might find helpful. West Penwith is perhaps the best (http://west-penwith.org.uk/), but there are lots of others, including St Keverne (http://www.st-keverne.com/).
Finally, there are the commercial sites, the national projects (FreeBMD for instance) and the Cornish Family History web site. I cannot speak for them, but you can easily find them via Google.