How do you make a forum website?
If you're new to the concept of webhosts and web scripts... then I'd suggest you the easy way out... www.smfforfree.com
Wait what exactly are you asking? As in how to install a forum? Is that what your asking?
1) Find a host (google is your friend)
2) Installing SMF (http://docs.simplemachines.org/index.php?board=2.0;sort=subject)
If you have any problems feel free to ask. It's fairly simple.