How to get a page before the blog opens ?

This is my blog : http://www.nifaladam.com
But i want a different webpage that i designed to open first (named : firstpage.php) and then there should be a button that says “Go to Blog” inorder to go to the blog. Kinda like the ads that come up when you visit some websites that says “Click here to go to the site”.

I want that to be done without changing the location of my wordpress from the root directory.

Maybe something like the firstpage.php opens before the index.php of wordpress and then by clicking “Go to Blog” i get to my blog. One other thing is that when i click the header banner or press home in my blog, it must take me to the blog and not the first page that i designed.

Any ideas on how to achieve it? I tried a lot 🙁