Cocktails can be shaken or built and its important to know what method you should use.
Depending on which cocktail you will be making, you can either build it or shake it.
Cocktails such as Margatia, Mojito etc. are shaken. The ingredients are mixed in a cocktail shaker and shaken with ice. Depending on the drink, it can be lightly shaked or heavily shaked. Margaritas and Mojitos are usually heavily shaked to provide a more cold and mixed flavor, Drinks such as Cosmopolitan often are lightly shaked to achieve that clear pinkish color.
Cocktails such as Tequila Sunrise are built. They are made in the glass itself with ice. Ingredients are just poured over. You can either stir (build and stir) it a little bit or not (build over ice) depending on the cocktail.
Before making your favorite cocktail, you can research online for correct preparation. There are so many varieties and twists. But all of them are great. Have fun mixing!