Players that have just started playing poker almost always have a ton of leaks in their game. It takes years to become a really good poker player, despite the common opinion of new players that they are already good. The most common mistake that new players make is that they play too many hands. They don’t yet understand starting-hand winning percentages and think they have an equal chance to win before every hand.
The other common one is to call too many bets when all they have is a draw hand. Novice players believe they will hit their draw too much because they don’t know the probabilities of the draw hitting. They don’t understand pot odds, and that calling large bets with drawing hands puts them in a negative wagering situation. And novice players play their cards too much, and not the player, which is rarely enough to win.