First look if the ones you have your webspace from also offer forums. Sometimes that's the case. You can also look at the bottom of this page. There's a link where you can get the base for a forum like this. It's php but if you know html it shouldn't be a problem.
Building one on your own with html is more complicated. You need to know about forms and you need to have access to some programs on the server that can use the data the user typed in. Better you ask if you can get this access before you start to write a forum. I'm sure you can also write one with other languages but there you better ask someone else than me.