If i buy this from GoDaddy if my domain name is http://domain.com will it secure http://domain.com/forum?
If you bought an SSL for http://domain.com then it would work for https://domain.com and any subfolder under it like https://domain.com/forum . However, it would not work for https://www.domain.com or https://forums.domain.com . Just a note, the domain name which you install the SSL must come with a dedicated IP address.
Eddy was only half right - you need a dedicated IP address *OR* a dedicated port! Either one will work. However, you can only get dedicated ports on a dedicated server (NOT shared hosting). Since I have dedicated servers, I use dedicated ports, myself - saves me a lot on buying IP addresses!