While this might be not the easiest task for a beginner, it is very well possible – with a little help from some plugins.
In a similar scenario I would usually install WordPress under a subdomain. When everything is looking ok to launch, I would recommend to first (always!) make a backup of everything. You can then change the base url (domain) either through the admin area in WordPress, which can be a bit tricky because once you switch you will get an error message. And you’re logged out. But that only means that the domain was switched. I actually prefer to change the domain name via phpMyAdmin directly inside the options table instead.
Finally there is one important step to take: WordPress writes all the links (inline page links and image urls) including the domain. So your site may first look a bit terrible, since no images are found – and inline links point to the old (sub-)domain. Using a plugin like Better Search Replace can be of great help to find and replace the corrupt urls. You simply have to replace the old url string (e.g. “http://subdomain.example.org“) with the new url string (e.g. “http://example.org“). That should do.