There are many different sites that offer blackjack online and several variations of blackjack on those sites. Each variation tends to have its own rules. However, there are basic blackjack rules that everyone should know before playing even a free version of blackjack. The common rules are incredibly simple. Once you bet, if you're not playing for free, two cards are dealt, face up. If your cards equal 21, you win. If the dealer also has 21, it's a tie. If neither you nor the dealer have 21, you have the choice of hitting, standing, or surrendering based on the cards in your hand. The cards numbered 2 to 10 are worth face value. The face cards (Jack, Queen, etc) are worth 10 points. An ace is worth 1 or 11 points, whichever works best for your hand. When an ace is used an an 11 it's called a soft hand. When an ace is used as a 1, it's called a hard hand.
Some strategies that may prove helpful to you are always hit when you have a hard 11, or less. Always stand when you have a hard 17, or more. Always hit when you have a soft 17, or less. Always stand when you have a soft 19, or more. You should always stand on a soft 18, unless the dealer has a 9, 10, or Ace, then you should hit. You should always stand on a hard 12 if the dealer has a 4 through 6. If the dealer has anything else, you should hit.