There are several ways to monetize a forum. Most of them are related to the effective use of Advertisement, Sponsoring or Charging membership fees.
You can use ad networks like Google Adsense, DoubleClick, Media.net, BuySellads.com, Sovrn, and many others, which give you the chance to place ads with ease in your forum and monetize base on clicks to ads. Though your revenues per ad are small, so large traffic is required in order to monetize.
Sell directly space ad to companies.
You can sell ad space directly to companies and agree on a CPM or CPC fee, this will require for sure more work, but revenues can be much higher and you can select the ads which are more suitable and aligned to your forum.
You can mention and promote products in your blog, once you build trust in your audience you can publish an affiliate link. There are platforms like CJ Affiliate by Conversant (formerly Commission Junction) which allow you to automate this process and enter into CPC, CPM or CPS structures, or you can contact directly brands that are having the same target group as your forum.
Invite key opinion leaders to answer questions in live sessions, where they obtain visibility and awareness and your forum will get their questions answered, this is a win-win structure which might help you to gain trust and become more relevant.
If your forum is bringing a niche together, and there is a big value of being there, you are able to charge membership fees, and form a membership program which will generate attractive revenues.
Other ways of monetizing
You can explore building up an Online Community with platforms like Connexa that already allow you to monetize your community, and establish a sustainable online space.