Online blackjack is a quite simple game. If you can count to 21, you're already on your way to learn the game.
The aim of Blackjack is to get a larger hand than your opponent's hand. However, your hand may not exceed the magic sum of 21. Your opponent in Blackjack is always the dealer. As a result of this you should only pay attention to your own and the dealer's cards. Unless you're interested in some of the more advanced Blackjack strategies, you don't have to care much about the other opponent's cards.
In Blackjack it's the face values of the different cards that matters and not the colour of the cards. If you get an ace, this card will be worth 1 or 11, and the ace is always considered to have the value that makes the best hand. If you are dealt a ten, jack, queen or king these cards are worth 10. The rest of the cards are worth their face value.
In Blackjack you'll get two cards to start with. After you've received your two cards, you must choose between "hit" or "stand". Chose hit if you think you'll get closer to 21 by getting another card. You can hit as much as you want, but if your cards exceed 21 you'll lose. Choose to stand if you're as close to 21 as you think you can get. Choosing "stand" means that you'll get no more cards.
If you get two cards worth the exact sum of 21, you have a Blackjack, and you don't have to make any decision at all since this automatically makes you the winner. However, the dealer may also have a Blackjack. If this is the case, there'll be no winner.