A deck is made up of a minimum of 60 cards and has a sideboard1 of a maximum of 15 cards. In your deck you can have any card that the format you are building your deck in allowed for but only up to 4 of any one card (except for basic lands which I'll get to later).
The object of the game is to bring your opponent's life total to 0. In most formats their life total startes as 20 but in other formats such as Commander EDH your life total startes at 40. To bring your opponent's life total to 0 you have a variety of different options. You can attack them with creatures or burn them with instants and sorceries. There are even some cards that say straight up you win the game. Regardless of what you choose you will need a deck to do so. On the Getting Started page there are 5 cheap pauper decks you can get so you can jump right in.
1 The sideboard is an optional set of cards with a 15 card maximum in addition to your main deck that you can use to defend against decks that you have trouble winning against. Tournament matches are played in a best two out of three style, so when you have finished your first game against your opponent you can exchange cards from your main deck with with the ones in your sideboard to help you win against the specific deck you are facing. (More in the Resources section)