For starters, your WordPress website will have a current theme that will style your home page, posts, pages and other such navigational pages like categories and archives. A good reference to see what exists in a typical WordPress page can be found at the WordPress.org website theme development.
Please keep in mind that many custom themes that are free or paid may have their own custom named files that handle specific requests; such as a page that lists of posts of a specific category, or archived pages based on the date.
To edit the pages, you need to know where they are. You have two means of editing the content; one through the WordPress backend and the other from editing files within the wp-content/themes/mythemename folder.
Method #1: WordPress backend
If you plan to use the WordPress backend, you can find them at Appearance >Editor. After you select ‘Editor’, the pages will be displayed on the right hand side of the screen. Do note, that some themes can extend another theme and that means you need to go to a parent theme in order to make your desired changes.
Once you make the changes, you can simply click the ‘Update File’ button of the bottom of the page that you are editing. If you plan to edit content this way, I recommend backing up the wp-content folder or file in order to have a disaster-proof plan.
Method #2: Files From Directory
If you navigate to the wp-content/themes/mytheme(your actual theme you are using) folder, you can edit any of these files as you want. Thus, you can open your favorite editor and code away.