Poker is a game of extracting the maximum value from winning hands, and minimising losses from losing ones. This is known as Min-Max. In order to win money you need to have a good enough hand to justify your call, but also be able to keep the opponent off of a better one with your bluff.
Professional players make use of their knowledge and experience to create a strategy that suits them. This can be done through careful self-examination of previous games, notes taken at the table, or even by discussing their play with other players to get an objective look at their weaknesses and strengths.
It is not necessary to learn complicated strategies or memorise a system of play, but it is important to develop quick instincts by playing and watching experienced players. This will help you to spot mistakes other players make and improve your own play.
For example, experienced players will often raise instead of limp when they have strong value hands. This is because raising prices all the worse hands out of the pot, and helps to inflate the overall pot value. Conversely, weaker hands are more likely to be called if the player limps.
Other strong players will sometimes bluff when they have weaker hands to deceive their opponents into believing they have a strong hand. This can be an effective way to make the opponent believe that they have a weaker hand than they actually do, and force them to fold before showdown.