The CPanel File Manager should look something like this:
The lefthand menu looks a little daunting, but most of this is system stuff you needn't touch. You need to navigate to the directory/folder which contains your website. On Siteground this is public_html. You may find that is the case already. If not, select it from the lefthand list. Incidentally, the 'www' item is a shortcut to 'public_html' and it doesn't matter which you use. On some other hosts the website might be in a folder called 'htdocs.'
If this is a new account, public_html will typically have just one or two files in it. You should delete any default.html or index.html file. This is just a placemarker so that your website will show a message when opened in a browser, rather then being blank. If there is a .htaccess file, you can leave that.
Next, you need click the Upload button.