The Fool speaks of new beginnings and adventure. A new light will be shed on old things. The Fool is young, lucky and light-hearted.
The Fool is the first card of Tarot but is originally not numbered. Some decks have it marked #0, some traditions consider it the last card of the deck. My advice to you is to decide for yourself based on your instinct and relation with the cards you work with.
An interesting approach is to consider him the protagonist meeting all other characters of the major arcana. Taking a leap of faith, he goes on an introspective journey.
The Fool stands for new beginnings. As he wanders the world, he shows humility in the face of creation and asks himself questions about love, justice, evil and more.
Must know: depending on your deck, the Fool may be more or less shady. In the Rider Waite deck, this card is way lighter and more inviting than in the Marseille Tarot for instance. This can affect your reading and the energy of your spread.