In blackjack you must often decide between drawing another card (hit) and sticking with the cards you already have ("stand"). There are several ways to inform the dealer about your decision.
With standard blackjack, you can indicate that you would like an extra card by drumming on the table with your fingers. However, sometimes it is better to just tell your dealer what your decision is in order to avoid misunderstandings and make intentions absolutely clear. If the value of your hand exceeds 21, then the dealer will inform you that you are "busted" and he will take you take the money you have gambled. If you decide to "stand" then you should hold your hands above your cards or, again, just speak to the dealer.
There dynamics are a bit different with a face-down game. Players hold the first two cards in only one hand. If you want another card, then you can "scrape" your cards on the table. If they value in your hands exceeds r 21, then just ‘throw’ your cards on the table and say nothing. In the case of 'stand' you can put your cards (face down of course) under the money or chips.
If you are unsure about how to act, then you could of course always observe the other players how they do it.