You can add your own themes and plugins directly from within the WordPress dashboard. Once you login to your dashboard, click on either 'Appearance' or 'Plugins'. Appearance lets you change between themes, and 'Plugins' lets you control, activate, or deactivate plugins.
Next to the 'Themes' or 'Plugins' header, there's an 'Add New' button. If you click on that, you can search the public WordPress.org plugin and theme libraries, and simply click 'Install now' against the theme or plugin you want. Alternatively, if you've a plugin or theme in a zip file, click 'Upload Plugin' or 'Upload Theme' respectively, and then browse to the file. After clicking upload, you'll see the plugin or theme in the right area of your dashboard.