How am I supposed to build up a population on my website?

[Alexandru]

You can make some traffic using directcpv.com is a website where you pay for each people is coming to your website and it is very cheap. You can pay google to get you some targeted traffic but it is going to be very expensive. You can also pay facebook to drive you traffic to your website but you have to pay for that.
Or if you have a twitter account or facebook account just let your friends know that you have a forum and tell them to join the forum.
Or you can write articles with your signature on them and post on other people blog, you can get some traffic from craiglis, register on forums and make publicity of your forums buy leaving comments over there don't forget to have the website link on your signature.
You can write some article and post them in article directory like eZine.
If you like to make movies just put your movies on youtube and put your forum link over there.