Apollo is a younger god, the son of Zeus and the nymph Leto, and the twin brother of Artemis. The Greeks often thought of Apollo as being the same as Helios, the Sun god, or the same as the sun, and so he is one of the sky gods who always beat out the earth gods in Greek myths. Apollo's younger brother is Hermes.
Apollo does not marry or have many children, though sometimes he falls in love. Apollo is a wise god who can tell the future, and his temple at Delphi was a famous oracle, a place where people went to find out what was going to happen. One of his sons is Asclepius, the god of medicine. Apollo is also a musician who plays the lyre.
The Greeks told a story that when Apollo first came to Delphi there was a great snake living there, a sort of dragon, the Pythia. Apollo killed the dragon and that was how Delphi became his temple. This might mean that there was an earth goddess who was worshipped at Delphi before the Greeks came with their new god Apollo. It is a lot like the story of Medusa.
To find out more about Apollo, check out these books from Amazon or from your library:
D'aulaire's Book of Greek Myths, by Edgar and Ingri D'Aulaire.
Greek Religion, by Walter Burkert (reprinted 1987). By a leading expert, for adults. He has sections on each of the Greek gods, and discusses their deeper meanings, and their function in Greek society.