Archives tend to hold very, very large numbers of records, so you'll need to target your research by establishing a much narrower range of interest than the entire collection of any particular archive.
Once you've worked out what sort of records will be useful to your own research you'll be able to request access specifically to these.
Searching archives is usually done through finding aids such as catalogues, which include descriptions that help researchers pick out the records likely to be of interest. You may already know of at least one set of records of interest, if that's what led you to plan a visit to a particular archive.
Many archives make all or part of their catalogues available online, so always check an archive's web pages first of all. If you can't see a link to archival web pages from the organisation's homepage, try looking for library pages as the archives may well be held within the library. Try to do as much online searching as possible before you actually visit an archive as a computer terminal may not always be available to you when you visit.
However, remember that some archival catalogues may only be available in hard copy, e.g. in book form or as a card index. This may apply either to all of an archive's catalogue or only to certain parts. When you first visit an archive, ask the archivist on duty at the service desk what finding aids are available for the sorts of records you wish to consult.
Archival catalogues are usually hierarchical, with less detailed 'umbrella' descriptions sitting above those descriptions relating to particular parts of the collection. A catalogue entry is likely to contain details of the source(s) from which the records were received by the archive and their dates of creation or acquisition, as well as (hopefully) an outline of the content.
The next tutorial is Interpreting documents in archives.