WWI is the short written form of World War I — the war fought mainly in Europe from 1914 to 1918. The 'I' here is the Roman number one, so you read the whole thing aloud as 'World War One'. Nobody says the letters 'double-you double-you eye'; the abbreviation is for the eye, not the ear. It belongs to writing where space is short: history notes, timelines, museum labels, exam questions, chapter headings. In an ordinary sentence in a real essay or article, most writers spell it out as World War I or the First World War. British writers tend to prefer 'the First World War'; American writers use 'World War I' a bit more. And note that people only started calling it 'the First' once there was a second one — before 1939 it was 'the Great War', which you will still see in old books and on memorial stones.