Installation / hosting prob

Hi all, inexperienced newbie named Pete here.
I have just arranged my hosting through a company called heartinternet.co.uk, first impressions – very good. They offer one click installation of wordpress.
I am planning to build my wordpress site to become the main site for a forum that I started a while ago that now has 1200 active members. The domain that I want to use for the site is the domain that is currently used for the forum. The plan was to let the forum keep using the domain whilst the wordpress site is being built and when complete then point that domain to the completed wordpress site.

The trouble being (my inexperience) is that I cant seem to start the wordpress install without setting up the domain. I did start to go down the route of a ‘domain reference’ which as I understood allowed my domain to still direct traffic to my forum whilst the wordpress site is built but when I was prompted to point the current name servers to heartinternet I got spooked??? will this redirect the normal forum traffic to the pg thats being built or is the host just wanting to become the agent for my domain?

How do I install wordpress without disrupting the forum?

Thanks in advance

Pete