There are many, many ways to monetize a website; to earn an income, even a substantial, passive, income from a website. Even if you're not marketing anything, it is possible. Large numbers of for instance charity sites nevertheless earn handy income by merely existing.
The way to achieve this is to first develop a lively website that attract large numbers of visitors. Once that is running smoothly, you (by example) indicate spaces on your website where you will allow advertisements. Google will then place AdSense ads in those areas, correlated with adjacent keywords in your text. Every visitor that leaves your site by clicking on one of these ads, will earn your site a small commission. When large numbers of visitors do that, the income can become substantial.
Monetizing is a very large field, with literally hundreds of ways to achieve it. Do not fret too much about it at this stage - the SBI! Action Guide address this subject in satisfying detail, and you will only be implementing it once you've created at least 30 to 40 high quality, rich in content pages on your site.
But that does not mean you can't think about it so long. Following this section, I'll supply a regularly updated but non-exhaustive list of some of the more popular ways to monetize. Visit their sites (I've no association with them in any way), see what services they offer, get your mind around it, get used to the idea: Tastefully used, such options can be excellent, passive income earners:
LinkAdage and Live Customer impressed me the most
Text Link Advertising
(You get paid for allowing selected advertisements on relevant content pages)
Text Link Ads (aff.) – The leader in paid text link ads.
LinkWorth – Keep up to 70% of the ad revenue.
Text Link Brokers – Make money buy selling links in a directory on your site.
Live Customer – Static HTML ads with flat monthly rates.
LinkAdage – Matches advertisers with publishers
Direct-Link-Ads – Keep up to 75% of the ad revenue.
PaidTextLinks – A smaller text link program.
AdSter – Customize your ads to match your website.
Vibrant Media – Vibrant in-text advertising.
TextAd.biz – Basic text ad service.
I'll be looking at this next: