I will outline what are the greater and lesser virtues of poker and expand a few areas. Note that these deal very little with how you play certain hands and much more with the ethos you should bring to any table you play at and which tables you chose to sit on.
The standard of play at low levels is sufficiently low that you dont need to be good to win. But if you approach things in the wrong way - unless you are a poker genius you will be unlikely to suceed anything like as well as you are capable of.
Respecting your bankroll is important but given so many start playing for money and giving their bankroll such little respect I felt it was worth emphaisising and elaborating upon.
Dont play at a level out with the swings you can cope with if you are playing 5$ sit and goes then 50$ bankroll is probably not quite enough to cope with the swings. Dont play in long odds tourneys for money at all. Wait till your play improves to a level where you might be able to win and your bankroll improves to a level where entering such a tourney will not make any noticable affect to your bankroll.
Also respect yourself - dont allow yourself to go all in against someone in a tourney when you only have a tiny edge (52%) if you feel you will get a better chance against them later. Particularly if you are playing in say a 5$ sng and have less than 60-70$ in your account.
Another key concept is the ability to adapt to the different style of games that you play. If you are jumping from money to freeroll to money to freeroll that is fine but you must realise that you will have to play in quite different styles to be successful in these different games and if you are not able to adapt then dont jump about between the two.
Also if you have misplayed a tourney dont go straight into another one again you are risking you bankroll when you are playing at less than your best and you could start something that will develop into a tilt.
If you are playing for money then keep records. Remember each time you win how many 50/50's you won to get you in the money etc if it takes 2* 50/50's and a 70/30 to get into the money that is fine but if that happens when you are playing your best game which you dont often play remember that this was only worth (say if first is 45 and entry 6 and otherwise you would be out) about 9$ profit with average luck - very decent but only if you can maintain this consistantly.
As Caro says you are playing to make the right decisions, not to win touneys and pots - these will come if you make the right decisions in due coarse anyway. You will also notice which times of day are more profitable for you and this enables you to play more at these times and maximise profit.
If you respect the poker virtues then you will be able to sustain and strengthen your bankroll without needing to play that well. It requires discipline but those who dont apply it are paying to play poker - they may not mind, they may feel that the rakes are just their fee for an eveneings entertainment. But they are paying my entrance fee and enabling me to make a profit why dont you join me rather than them. You dont need to play well you just need to play wise.
The Skill in Holdem Poker is Pre-Flop Analysis - This involves deciding using statistical odds the strength of your hand.
Ace King (Suited)
Ace Queen (Suited)
King Queen (Suited)
Ace Jack (Suited)
Ace King (Offsuit)
2 7 (offsuit)
2 8 (offsuit)
3 8 (offsuit)
2 9 (offsuit)
2 6 (offsuit)
Then you have Pot odds and Implied pot odds, these are used when you are slightly more advanced player, and they help you make judgements on whether you should call raise or fold based open bets made and the number of players currently in the hand.
After the Flop, turn, and river you have to make decisions about the stregth of your hand, calculate the number of "outs"(number of possible ways to win the hand) you have, and also what your opponents can have which will beat you.
e.g, Say you hold Ad-Kd (Suited). And the Flop comes Ac-Kh-Qs. You have a strong hand - You have Top Two Pair - But what can beat you? Well 3 of a Kind Aces, Kings, Queens. Also a Straight 10-Ace
Turn Comes Ac-Kh-Qs-Qc. You Still have a good Chance of Winning the Pot, But you now have More possiblities that can beat you.
3 of a Kind Still Beats You
a Straight still Beats you
4 of a Kind Beats You
Full House Beats you, e.g A-A-A+Q-Q or K-K-K+Q-Q OR Q-Q-Q+K-K etc.
Flush Draw is on the Cards
You have to make use your skill as a poker player, using the way in which your opponents bet, do they fast call? Slow call? check-raise etc. As well as the Size of Bets that are being made, If someone is making a large bet, and if there are a few people still in the Hand its likely someone has a Full House or three of a kind or a straight.
The Only Luck in Poker comes really when two players go All-in Pre-Flop or post-Flop and a player who is at a significant disadvantage statistically outdraws the player who has favorable odds. e.g I hold A-A and another player holds 2-3 he raises all-in and i decide to call. I have a large statistical chance of winning. However the flop comes A-4-5. Now he has made a straight, and i am dead unless i can catch a Ace or a Pair on the Board.