I installed the silverstripe and started working on it. Later on I have been told to put a splash page so if public visits the domain we will be able to present a simple form for subscription. Now, this page is not done with silverstripe, it is pure php file and has a process script to handle the form submission, basically seperate from the main site. What I would like to do is, only redirect user if they visit the domain, and still be able to access to /home/ and other pages that I have been working on.
Before you waste any more time on this, the advice you have been given is bad.
In most cases, splash pages are terrible for SEO (Search Engine Optimization) as they lack decent content that search engines look for, and are rarely the type of pages that other sites want to link to (and you want sites to link to your root domain).
Have a read of this and try "splash page seo" in Google for more writeups on this very old practice.
If you are really decided on doing this, just say so and someone will be able to help you with the tech aspects of the problem. But I highly recommend that you don't use splash pages on the root page of your domain. ESPECIALLY if it contains flash.