Can anyone explain to me when and where to use articles? I always get confused with the usage of a, an, the.
Very briefly, 'the' is the definite article and it's placed before nouns when both speaker (or writer) and listener (or reader) know what is being referred to. The indefinite article, 'a' or 'an', is placed before nouns where that is not the case. 'A' is used before nouns whose pronunciation begins with a consonant sound and 'an' is used before nouns whose pronunciation begins with a vowel sound.
There is, however, rather more to it than this (for example, we sometimes don't use an article at all). If you are a non-native speaker there are full explanations in text books for foreign learners of English. There is also a useful tool to help you decide which to use here.