To become a good Poker player:

Learn the ranks of the Poker hands thoroughly.
Learn the relative values of the hands and what hand would be expected to win the pot in a given type of game.
Learn the odds against winning with any particular hand, and how to figure the odds offered by the pot.
Observe the other players in the game, to learn their habits and to read their probable strength or weakness from their actions.
Avoid giving away your own habits, strengths, and weaknesses.
Learn how to bluff.

Don't give away a good hand by betting too eagerly right away. For example, if you have a great hand from the start, don't raise the pot so high that it scares people into folding. You want the pot to build and the other players to raise their stakes so it will be harder for them to fold.
Note Again, this tip has less impact in games with structured betting. Many times, a good hand actually has only a small chance of improving, so it can also be a good strategy to win the pot as early as you can.

Your odds of winning a hand increase as the number of players decreases (through folding).