Members of Parliament (MPs), who sit in the House of Commons, are chosen by voters in general elections, which the UK government says usually take place every five years. There are 650 MPs, one for each local area called a constituency.
MPs are elected using the first-past-the-post system: each voter casts one vote for a candidate in their constituency, and the candidate with the most votes becomes that area’s MP, even without an overall majority. According to GOV.UK, to vote in a general election you currently must be registered, be 18 or over on polling day, be a British, Irish or qualifying Commonwealth citizen, and live at a UK address or be registered as an overseas voter. Members of the House of Lords are not elected this way.
Source: GOV.UK, Types of election: General election
Image: Houses of Parliament, London, by Adrian Pingstone, public domain, via Wikimedia Commons.