The first step to playing Minecraft is to download Minecraft!
You will need to create an account and purchase a copy of the game. Once you have purchased your copy you can go to the download page and hit the big green DOWNLOAD button.
After downloading the client double-click the icon to run it.
Once the installation is complete you are ready to go! Clicking on the game's icon will take you to the start screen for the game.
You can always download the game again if you need to use a new device by logging in and following the same steps. You won't have to pay again!
Ensure that you have linked your Minecraft purchase to your email address and not to a device!