Forums use to be the tribes fire, now something between some to most are abandoned. not because they're niche is not good, not because the software it uses (XenoForo, vBulletin, phpBB, etc), it's because the all concept has been replaced by social networks.
Myself was co-owner at one of the largest forums in my country. even though we didn't like it - our most active forum was MU Online on our online games section, kids actually paid 5$ via SMS to sign up for that forum. what happen to it? I died slowly and painfully until it was shut down completely at 2014 (existed since 2001).
There are forums who are still active and alive, but it takes years and lot's of investment to make them work, if you have spare time and money and you have that urge - then ofc go for it.
To the point :
start by making 2-3 friends to be active daily (the hardest part...).
don't open up sections which you think might not be very active at first, be more general.
try to find medium-active participants in other forums and try to invite them to see your forum (without breaking any rules), they might stick around (you can offer them to become moderators if they qualify).
advertise in as much website directories as you can, invest a fixed monthly amount for advertising, like 10$-20$ a day and choose your adv platforms wisely.
try to publish helpful content yourself in sections.
be a gent and be fair with your users, answer their PM.
stay up to date with upgrades and plugins - but don't change your design often (or at all). people don't like changes.
Wish you best of luck!
