The game of poker is a combination of art and science. The science is being disciplined to stick to the best practices, while the art is knowing when to break those rules and make adjustments on the fly. This is especially important when it comes to reading your opponents and their tells (non-verbal cues that signal whether they are bluffing or scared).
A hand must consist of one of the following combinations: Two pair: two cards of the same rank, plus two unmatched cards. High card: This is used to break ties in the case where multiple hands have the same pair or higher. Three of a kind: 3 cards of the same rank, plus 2 cards of another rank. Flush: 5 cards of consecutive rank, all from the same suit. Straight: 5 cards of consecutive rank, but from more than one suit.
Once the first round of betting is complete the dealer will put 3 more cards face up on the board that everyone can use, this is known as the flop. A new round of betting begins, again starting with the player to the left of the dealer.
The player who has the highest hand wins the pot of chips placed in the betting. The dealer will usually announce the winner, though it is common to leave the decision to the players themselves. This is especially true if the player who won has a superior hand than any of the others.