You should advertise your forum on internet at first. Let people know about your forum, the benefits for them joining your forum. Then they will register automatically.

contribute in relevant forums, tell people what your forum offer. make a explainer video and upload it to youtube and other video sharing sites. use social media to promote your forum.

Or you may also create accounts on social media sites (facebook or twitter) or perhaps create a good YouTube video promoting your forum site. You can also try to run a paid ad on Facebook to get more likes.

Getting a forum off the ground is different.

Even if you managed to get it ranking #1 for high search volume keywords people aren't going to sign up if no ones posting on there.

You need to market it first. Hire people to post on it, advertise it on other sites/blogs etc.

You should focus on getting a couple of hundred active users before worrying about SEO at all, in my opinion.