Maintenance mode using custom html

Hello, I need your help if possible. I want to use wordpress to create a website. At the moment i have just created an custom html page with the name index.php thats comes up when someone gets on url. I want to upload wordpress CMS and work it BUT i also want the visitors to still see my custom html page untill i remove it.

So first of all how can i ahve the custom html page always to show up by default and what changes i must do when i want to publish my site and remove the custom html page.

Thx in advance,
Chris