Generally, there are two sources of income available when you monetize a site; the site user pays you or, a third-party pays you. You can use either or both of them.
The site user pays you:
You must offer something of value for the site user to pay you. Subscription services or access to something of value.
- You could start a newsletter with related content, (paid) offers of value and premium extension articles. Subscribe to the Windows Secrets newsletter for a great example of how this can work. EDIT: Since December they no longer seem to have the free newsletter. I think this is their mistake.
- You could have a premium access section of your website, it can be independent or linked to the premium content of your newsletter or, it could be an API.
- Some websites only give the current information to subscribers, everyone else gets information hourly for free. This may work for market data.
Third-party pays you
When a third-party pays you, you give them something of value. For a website this is essentially for access to your site users but may also include for access to metrics or, subscriber details.
- You could install on-site advertising from one (or more than one) of many sources, you are generally paid per-click or per-view.
- You could setup referral programs. You could setup an entire page dedicated to quality 'Partner Programs' and earn from the referrals. Partner program articles often make for great newsletter content, one or two depending per newsletter amongst the other articles.
- You could opt-in sell (subscriber accepts third-party offers) qualified leads from your site-subscriber and/or newsletter-subscriber lists.
- You could sell advertising space in your online and/or email newsletter.
The other option
You could voice for donations. This is effectively voluntary-pay-for-content. "This site cost money to operate, if you appreciate this service please consider a donation..." Or, a donation banner drive with email-out. Wikipedia and Internet Archive both use this strategy to good effect. To do any sort of email-out you need a list. People can be very creative with how they call for payment, I have seen website signup processes with a voluntary payment as a part of the web signup.
Whatever you choose, I would recommend that you take the time to develop a quality regular newsletter, whether it is email or online, free, paid or, partial paid. This builds your reputation as an authority in your field and builds report with your site users.