Aces are high. If they elect not to do so, or if the card they draw cannot be played, then they must pick up the entire discard pile and add it to their hand. Players must keep at least three cards in their hand. So once a player plays some cards, they have to take cards from the top of the deck to bring the number of cards in their hand up to 3 again. And if there are no cards left in the deck, you of course don't draw anything even if you have less than three cards.
Play continues around the circle.
Once a player has disposed of all cards in their hand, they move on to selecting and playing cards from their top hand. As with cards played from their hand, two or more cards can be played at the same time if they have the same face value. If you can't play from your top hand, you pick up the discard pile, and must get rid of all the card in your hand again before you can return to playing your top hand. Once they have no other cards in their hand to play and all three cards of the top hand are gone, the player moves on to playing cards from their underhand. To play a card from the underhand, you pick one, pick it up, look at it, and play it if you can.