Well firstly, you can enable Adsense and then display Google Ads on your website. Each click will generate a bit of income and over time, the amount per click earned will be more as advertisers will pay more to appear on your website. Another way is to use a site like BuySellAds. This is basically selling ad slots on your website for people to advertise their website. This can be a good way to make money but to be able to do this in the first place, your site needs to be big.