ok here are the steps to do it properly :).
First of all login to your domain control panel and change your domain's nameservers to that of your hosting.
Now your domain will be pointing to your host. You want the domain root to point to your blogger blog. So we have to hand over the root domain to blogger. To do this we should create a CNAME record for www with value ghs.google.com
You wont be able to create this CNAME(unless you have paid more for some VPS or some hosting which gives you the permission to do it by yourself).So ask your webhost to create such a CNAME for you.When your host creates such a CNAME for you, your domain root(www) will be hosted at google's servers. So you can switch your blogspot domain to the customdomain http://www.yourdomain.com
So now you are done with your blogger blog part.
Next you need the forum on your subdomain.To do that login to your hosting control panel and add the subdomain forums.yourdomain.com and install some forum script there..
I hope this helps. I already have a similar kind os setup for my forums and blog. :)
Additional Stuff : you can enable google apps on the domain if you ask the host to create necessary CNAME A and MX records needed for google apps :)