Look up the optimal play for any hand against any dealer upcard.
Basic strategy is the mathematically optimal decision for every possible hand in blackjack, calculated by computer simulations of millions of hands. Following perfect basic strategy reduces the house edge from 2-5% (for average players) to approximately 0.5%. It is the single most important skill for any blackjack player.
Always split Aces and 8s. Two Aces give you two shots at 21. Splitting 8s turns the worst hand (16) into two playable hands. Never split 10s or 5s. A 20 is already strong. A pair of 5s is a hard 10, which is a great doubling hand. Double on 11 against anything except dealer Ace. Double on 10 against dealer 2-9. Stand on hard 17+. Stand on hard 13-16 against dealer 2-6 (dealer will bust). Hit hard 12-16 against dealer 7+ (you need to improve).
A soft hand contains an Ace counted as 11. These hands are valuable because you cannot bust by taking one more card. Generally: double soft 13-17 against dealer 5-6. Stand on soft 19-20 always. Soft 18 is tricky: double against 3-6, stand against 2/7/8, hit against 9/10/A.
Card counting is a legal technique that tracks the ratio of high to low cards remaining in the shoe. When the deck is rich in 10s and Aces, the player has an advantage and should bet more. The most common system (Hi-Lo) assigns +1 to 2-6, 0 to 7-9, and -1 to 10-A. Card counting can give the player a 0.5-1.5% edge, but casinos actively discourage it and may ask counters to leave.