How to customize themes in WordPress
In the admin panel of your WordPress there are some options for customizing your current theme. The number of options and the degree to which a theme can be customized using these settings depend on the particular theme. For any further customizations changes to the code of the theme's files will have to be performed.
To see what options your current theme offers and to configure them, log in to the admin panel and click on the Appearance menu on the left. You can customize the active theme. Under the name and description of the active theme you'll see a Customize button, as well as some other buttons such as Widgets, Menus, Header, Background, etc. The available buttons depend on the theme. The same buttons are also available in the menu panel on the left side of the backend.
Most themes have a Customize button which opens another page with some theme options. At the very least most themes allow you to change the site title and tagline. On that Customize page there may be some other options. There may be options for changing the background color, for removing/changing the header image, for setting a static page to be the homepage of your site's frontend, etc. If you make any changes, don't forget to click on the Save & Publish button at the top left corner of the page.
The Widgets and Menus buttons will redirect you to the sections in the admin panel from which you can manage the widgets and menus of the theme respectively. For more information read the tutorial on how to manage widgets in WordPress and the one on how to manage menus in WordPress.
As we mentioned, in addition to the Customize, Widgets and Menus buttons there may be some other buttons (e.g. Header, Background, etc.) for other theme options.
For some more details and screenshots you can also check out the tutorial on how to customize themes in WordPress.