If the ads are relevant to the content of the website, they can actually add value to your website. Any company advertised on this website is carefully examined for relevance and quality before it's displayed, so, basically, this is a win/win situation: the site makes money and the person who clicks on the ad gets the good or service s/he was looking for.
Your ads can be text ads or images like the ones displayed on this page. The most popular ads are Google Ads and Amazon ads. Google chooses which ads to display on your website. Amazon ads are more flexible. You can let them pick the products, or select one or more yourself.
You will need to dig in a little to the HTML of your website pages. Most online editors, like the one on WordPress, have a tab at the top of the editing box which allows you to switch to the HTML.
Another way to add advertising to your site is to join an affiliate program like Link Connector or shareasale.com. These programs sign up advertisers who supply attractive banners and linking code for you to add to your site. The procedure for adding the code is the same as above. The advantages to going through one of these affiliate managers are:
The thing is, any of these programs will require that your site be informative and have good content. This website makes money because there's a lot of useful information and it's frequently updated to ensure that it's accurate.
You'll find that, if you become visible in a subject niche, advertisers will start coming to you!