In the first place, the first mistake that an online poker player does, is to choose a bad online poker room. Do not choose a particular poker room because it has a Tv commercial, you just need to read a complete poker review. All of us were weak when we started to play poker. We all have made mistakes and we still do (some mistakes are made by players with thousands hands played at the poker tables).
One of the colossal mistakes that a player does is a bad Bankroll Management. The majority of the beginner poker players play at the inappropriate stakes for their bankroll. Out of curiosity, or perhaps because of an overestimation of their poker knowledge, many players raise the stakes although their bankroll does not allow it. Of course this means for everyone who makes this kind of decisions a pre-mature bankruptcy. If you respect the bankroll management you will avoid the Tilt, which is another mistake of the beginner poker player. What is the Tilt? Well, perhaps many players have heard about this expression, but not many really know what it means. The Tilt is a state of mind, the player which has the bad luck to enter in this state, will play chaotic and irrational for more than 20-30 poker hands. A poker player is tilted after he deals a series of bad beats or because he has a bad bankroll management. The most important thing is to take a break when you become tilted. The break can last an hour, a day or a week, depending of the physiologic profile of each player.
One of the biggest mistakes that the beginner poker players do is to play at many poker tables in the same time. This is a mistake, because all the players, which play 10-12 tables in the same time and win, are players with experience with over 200-300 thousands hands played. For the beginner poker players (up to 50,000 hands played) the maximum number of tables that they properly should play in the same time is 4-5 tables.