Well there are multiple ways you could try - One way is banner advertising, if you have a website with high traffic numbers this might be an avenue for you.
One way I've come across is Project Wonderful
It is mainly aimed at content publishers - The approval and set up process is relatively straightforward, requires a bit of work but can be fairly automated too so that once you are set up things run by themselves.
There is however the caveat that the advertisement platform provider arbitrarely might disable ad boxes etc, if they feel you do not have the numbers etc. that they need, I also found that multiple services providing banners and the like do not seem to produce the right numbers compatred to internal site counters as well as tracking links so I am somewhat in doubt about the banner strategy.
Offer services that you can provide from your site, these could be selling hours of work in your field of expertise, accepting donations in various forms. Offering people a platform where they can offer their unique services etc.
You need to put your heart into whatever way you choose and then you may see the results you seek, just remember there is no magic bullit even though sometimes you may wish there is.