Old English does not mean English from the olden days. Before Modern English there was Elizabethan English (like Shakespeare), before that was Middle English (like Chaucer) and before that, before the Normans brought a French influence to the English language, there was Old English. It's so different to what we now call English that it is really a separate language. Some call it Anglo-Saxon. We have to guess how it was spoken based on manuscripts and comparisons with other similar languages like German, Frisian and Swedish.
There are plenty of online guides and articles that teach you useful phrases in Old English .
Watch the videos below to get an idea of the different pronunciations and sounds used in Old English. For example, note that "sc" is pronounced like "sh", "ge" at the start of the word is pronounced as "yuh," and that all vowel sounds are voiced in Anglo-Saxon language.