As a rule between one and six decks of cards may be used depending on the Casino and whether you are playing a single player or multi player game.  The rules specify that two cards are dealt face up to each player.   The dealer deals them self two cards, one face up and one face down.  Card face values apply except for  picture cards which are valued as 10 points and Aces that are either valued as 1 or 11 points.  A "Blackjack" score  is an Ace and any 10 point card.  A push results if both player and dealer have a blackjack or both go 'bust'.    Players also win if their hand scores more than the dealer but does not exceed 21, in which case even money is paid.   If  21 is exceeded your are deemed to have gone 'bust' and you lose.

Should the first of the dealers two cards (the one face up) be an ace the dealer will check for "Blackjack' and ask if any one wants to insure their hands against a "Blackjack" score, for even money, to protect against a possible push result should the dealer have "Blackjack" too.  Only after all players have decided whether to insure or not will the dealer check the second card (the one face down).  Online dealers do not check for "Blackjack" consequently if the dealer has "Blackjack" you lose all your wager (including your double and/or split bet)

Depending on the card values you have you may decide to
    Stand (accept no further cards)
    Hit (request further cards - cards can be requested until you either stand or go bust)
    Double (Accept one further card having first doubled your original wager - doubling is
    usually only allowed on the first two cards)
    Split (Providing your first two cards are of equal point value they can be split into two hands.
    A wager equal to your original wager must also be made.  The original two cards now become
    the first card for each new hand - a re-split is the same procedure i.e. 2nd or 3rd split)
    Surrender & Late Surrender (Surrendering the first two cards is sometimes allowed after a
    dealer has checked for blackjacks, note - half your wager will be lost too)

Note - Online the dealers hand is usually hit until a point value of 17  - Soft 17 included.
(Soft 17 - that is an ace and any other card(s) valued to six points).

Check the rules of the online casino - not all options are offered i.e. some allow only one split,
most do not allow surrender.