This is a question I get all the time. And the answer is the same most of the time. In fact, you can replace with almost any other web property.
The answer is the same for all.
You have to provide valuable content for the readers and you have to promote the forum to the people that will benefit from that content.
Just because you build it, doesn't mean they will come.
If you have loads of cash, you can do what Neil Patel did with his forum when he launched it. Send $5000 cash to each of your friends and ask them to join and participate.
But chances are that paying would not work.
Neil has built some heavy duty social capital by providing huge value. He's built trust and a brand so when it came time to promote his new endeavor it was easy.
That's why he was able to pay for this. He basically paid for his friend's time to help him launch and multiply the trust and recognition on the forum.
But you don't have $50,000 or $100,000 to spend
So you have to go to the basics. The basics will always help you. Provide value. After you think you have provided good valuable content, then do it some more.
Then again. You do this by answering questions and letting the people that asked the questions know you've answered them.
This isn't rocket science nor is my prescription something that involves a lot of research. It's almost common sense. Look at how people are doing it, do some googling. Ask questions and then follow a guide or tutorial or the steps of someone that has done what you want to do.
Another friend, Matthew Woodward built his reputation this way and now he's got one of the most popular internet marketing blogs and forums.
Matthew simply looked for the people that would be his audience. He found what questions they had by understanding what problems they were facing. Then he would go to his website and answer the question in excruciating detail.
Of course, there are technical steps to doing things correctly, but these are technical, they're methodical and almost trivial. You can just follow best practices, but you have to have value. You have to provide content, solutions, entertainment.
Matthew's answers have been some of the best and people recognize this so they always go back for more. Matthew reportedly spends over 20 hours writing one of his legendary blogposts, sometimes twice that.
He once told me that he simply needs to outwork everyone else and the rest just happens, in other words...
Get off your ass and start working
It doesn't take rocket science, but it takes a lot of effort and consistency to achieve the trust you need to make a website property stand out. You have to do the legwork, you have to put in the time.
If you don't, you simply become like one more website or forum from the billions out there.
Get great content online and then show it to the people that need to see it. Do this and you will be ahead of the curve.
Installing a forum is trivial, starting a blog takes 5 minutes, running updates and changing settings can be done without any real thought.
Creating valuable content is the art and science that will help your forum succeed. As the saying goes content is king.
Oh yeah, and don't forget to promote what you write. Share it in social media, use your email list, tell people about it. You do have an email list right?