Where does your target audience hang out now? Those are going to be good places to issue your invitations to join your new forum.
You need to make sure you're offering something they can't get already in other places. Otherwise, there's not going to be much incentive for them to sign up for your forum.
You may need to advertise. You may also want to identify a few "key influencers" among your audience, cultivate a relationship with them, and send them a personal invitation to join your forum (or even ask some to become moderators, which can be a good way to make sure they're more active members at least at first).
You will likely have to spend a lot of time posting discussion topics yourself at the beginning. The idea is to have plenty of new stuff waiting to spark discussions when others show up. This is something you will likely have to continue doing for a very long time, unless/until your forum reaches "critical mass" and starts generating enough discussion threads on its own.
Be prepared once you start getting traffic to spend a large part of your time dealing with spam. Just a fact of life for forum owners / administrators / moderators.
Getting a forum off the ground is a lot of hard work. Don't give up too soon -- it can take longer than you think. But in order for it to be successful, you must
bring something unique to the table, whether it's a stellar team of moderators, or a different perspective on your topic, or a ton of top-notch information that can't be found elsewhere, or whatever. Otherwise, there's no real reason for anyone to join, much less come back regularly.