Hey, would i be able to set up my old computer, install linux on it and use it to host a forum site? what distro of linux?

sure thats what linux was built for good day.

ok... but what distro would be the best for that? and what programs would i use on it?

Any distro will do. Typically you'd like to pick a distribution well-suited to servers (characterized by long release/support schedules, very stable but somewhat outdated packages, etc...(Debian, CentOS/RHEL, ...)), but if you're new then Ubuntu Server or something like that will have you up and running quickly.

If you're doing a forum, phpBB is popular but there is other forum software as well (I think wikipedia has a good comparison of packages), and these will typically depend on Apache/PHP/MySQL so you'll have to set those up as well. You'll also probably want to look into dynamic DNS domain services like DynDNS in order to get a hostname.

It seems like a lot, but you will learn along the way, and with a good internet connection you can run a legitimate website out of your basement. One thing you'll want to keep in mind is that with a server, reliability is by far more important than new features or bells and whistles.

it is all linux. so if you want a big Gui interface then get suse with norvel. but if it is just for pushing packets and it is a slower machine then get slackware and learn how to do it from the bottom up. It is all linux it is nice to know in slackware you have control over the config files. all distro's do to. but when people get used to complete gui they tend to or at least I do . forget that manual configuration is a real nice thing. It just simply works.

You can use debian or debian based system for this. It's really easy to use. Also you can get help for debian more than you can imagine.