Identify the parish
Generally researchers turn to parish registers to look for a baptism, marriage or burial in a particular place having gained a clue to the parish concerned from other sources such as the census. The first thing to establish is whether or not that place was a parish in its own right or was in fact a township or hamlet in another parish.
As the population of the country increased rapidly at the beginning of the 19th century, many new ecclesiastical parishes were created from older ancient parishes. A good contemporary gazetteer should help. The Phillimore Atlas and Index of Parish Registers (Cecil Humphery-Smith, Phillimore, 2003) includes maps and lists all of the ancient parishes within English and Welsh counties together with similar information for Scotland.
It will give brief notes on the coverage of the registers and says where the original records are held. The Society of Genealogists produces a series of more informative guides to registers known as the National Index of Parish Registers, arranged county by county. All record offices produce guides to the registers they hold and nearly all have useful websites that include this information.
Before you dive straight into trying to decipher the handwriting in old registers, it is worth checking whether anyone has transcribed the registers.
It is usually easier to read a transcript than an early original, especially when the baptisms, marriages and burials may be recorded together, or baptisms were at the front of the register and burials recorded upside down at the back.
A list of all the Society’s copies can be found on its website and small booklets listing the various county sources in the library can be purchased.
Usually parish registers have been microfilmed or microfiched to preserve the originals and it is rare today to handle the old documents.
Whether you are looking at microform copies or printed transcripts it is important to keep careful notes of what years you have searched and note all relevant entries.
It might be difficult to remember much later whether you confined your search to one individual or whether you abstracted all entries of a given surname for a certain period. Make notes of what you searched for.