I am still a beginner, too, so I make a lot of mistakes, but stuff that I've done myself and seen done by others:
* not taking position into account
* not taking table dynamics into account - for example, when you are on the button or cutoff and the players behind you have vpip like <20%, you can widen your range and you will get the blinds a huge percentage of time
* not taking population behaviour into account - for example, on lower stakes it is relatively uncommon for people to
bluff with big bets. it happens, but if someone is betting pot on turn and shoving on the river it's most likely a premium hand
* not sizing your bets properly, especially when
bluffing - you will probably not make the second pair fold with a 1/3rd pot
* giving up too quickly - if you're c-betting, be prepared to c-bet all streets, especially if you think your opponent is drawing
* folding to any aggression - if someone is playing wide and c-betting frequently, they can be c-betting their entire range. check/raising them on the c-bet can be a good way to bluff them off of a draw, low pair, or ace high or sth like that
* playing too wide preflop
* not understanding or not calculating implied odds correctly when you're drawing
* overvaluing your hands, especially after a 3-bet preflop. If the board is low without any flush/straight possibilities or draws, and the preflop aggressor is betting big, there is a very good chance they have a pocket overpair. your top pair will probably be useless then
* assuming a player that plays very wide can't have a good hand. Like, you see someone with VPIP at 50% or more and you assume their hand is always garbage only to be beaten by AK, KK, or AA. Yes, statistically, you are ahead, but they can hit a premium hand anyway
* not defending blinds and folding too much in position. yes, you should probably not play too wide, but if you have VPIP at 10-15% you're playing way too tight
* defending blinds too much
* not analyzing your opponent's range when making decisions
* acting on impulse (probably one of the worst things I'm doing way more often than I would like) - when everything
tells you, you should fold, yet you still call on an impulse "screw it, let's see what you got"
* not paying attention to how the hand is played, ie. who is betting, raising etc.
* being afraid to 3-bet premium hands, and thus ending up in a family pot with people out-drawing your AA
* never 3-bet bluffing
* limping
* bluffing strictly because there's no other way to win the hand - sometimes you just lose a hand
* telegraphing your hand with bet sizing