To start customizing your theme, go to Design. On this screen, click on the blue button labeled Edit theme.
DO NOT click on Advanced > Edit code. This is for advanced custom theme development only and you will no longer receive any updates or theme support on newly added features. Theme updates can contain important bug fixes and new features. If you need to customize your theme, there is a lot you can do without activating the code editor. Please refer to this section for more details.
Read this sections to learn more about each options.