Hi Anna!

You can add a meta-tag in the head-section of your main template, like

    <meta http-equiv="refresh" content="XX" />

where XX is the desired interval in seconds.
You should consider that automatic refreshing of a main page with a lot of posts and active connections will generate also a lot of web traffic you have to pay!

Another possibility is a webpage which displays the latest posts rendered from the Forum's RSS-Feed (see Alex' method). An automatic refresh would make your day, but with much less traffic.