how do i start playing sims 4?

When you go to the Sims 4 section, there is a download button next to the Sims 4 logo . By pressing this, it will download and install the game. You can then purchase other packs if and when you would like to, but the basegame is free. When the basegame has been installed, then press the play button, and the game will start loading. On the start screen, there is an option on the left hand side to start a new game. Press that, and you can then start playing a new game. You can then create a sim in CAS ( which is the Create A Sim section ) and move them into a home in the world.
These threads have some other helpful information about playing the game 
https://www.ea.com/en-au/games/the-sims/the-sims-4/how-to-play-the-sims
https://help.ea.com/au/help/the-sims/the-sims-4/the-sims-4-gallery/
https://www.ea.com/en-au/games/the-sims/tips-and-tricks