Again, looking at useful religious archives advice from outside the UK, there is helpful elementary advice for starting to create a congregational archive where none exists is given at this Texas Baptists webpage:

http://texasbaptists.org/partners/texas-baptist-historical-collection/history-helps/how-to-start-a-church-archive/

It draws on these more detailed pieces from the Southern Baptist Convention Historical Library and Archive website http://www.sbhla.org/articles.htm

Although intended for a US and Baptist audience, much of the advice is more widely applicable.