Identifying useful archives
You already know the theme of your research, so the next step is to work out where records likely to be useful to you are held.
This is one of the most challenging points of your research: you know what you're looking for, but where do you find it. Matching an archival collection up to a research question can take an awful lot of lateral thinking.
Why was an archive established?
Typically most archives can be used for very different reasons than they were set up or collected for. Think of the census data: it is collected by governments for statistical and tax purposes, which is very different from its main use, which is family history. You could also use an archive of correspondence to find, for example, a relative's address, but the use you are putting it to is very different from the purpose for which it was set up.
Most archive descriptions inform you of the reasons an archive was set up, and may (if you're lucky) have subject headings which can help you find out whether the archive holds the sort of information you're after, but don't specifically address the issue of what uses the collection can be put to.
This web site is aimed at filling that gap, but there are other ways of trying to match collections to your research question: one of the best ways is to talk to other researchers who are interested in the same area you are, and share your experiences. The best way of doing this is to join a local or family history group.
It's best to do as much of this research as possible remotely (online or by email, 'phone or letter), to avoid wasting time and money travelling to archives that turn out not to hold useful material.
Try looking at the sources used by researchers who have undertaken studies comparable to your own and seeing in what kind of archives they're held, e.g. university collections or local authority record offices.
Unlike librarians, archivists tend to think in terms of types of record and where they come from (the records' 'provenance'), rather than the sorts of information they hold, & to organise their collections accordingly. Bear this in mind when making enquiries, e.g. ask 'Do you hold parish registers for the 1790s?'
If you expect a particular organisation's records to be helpful, contact them and ask whether they keep all their own records or, if not, where and at what stage their records are deposited elsewhere (e.g. transferred to the county record office after ten years).
Consult online indexes of archival holdings, such as The National Archives' National Register of Archives (NRA ). In their own words, "The NRA contains information on the nature and location of manuscripts and historical records that relate to British history." i.e. it will help you work out which archives are worth visiting.
Also consult lists of archival repositories, such as TNA's Archon directory. Archon offers information on access to the archives it lists, which will help you assess the practicality of making a visit.
The next tutorial is Searching archives.