In Texas Hold'em, the player who made the last aggressive action (bet or raise) on the final betting round should turn over their cards first after the river. If no bets were made on the final betting round, then the player to the left of the dealer button should reveal their cards first.
Can you bet after the river card is dealt? Poker allows betting on every street. Players can bet before the flop, after the flop, after the turn, and then again after the river. A player can check, bet, call, raise, or reraise after seeing the river.