Sign up with adsense. They will give you the code you need to past into your site. The adsense site is adsense.google.com
Your website is hosted on hosting domain? If yes, go to your hosting account, go to your ftp files and edit all your files by adding your code. If your site is a blog like Wordpress, you can add your code in the sidebar in your dashboard.
If you have already adsense account, just copy the code and paste it in your site on the place where you want to display for.
What platform are you using? Pure HTML site? Wordpress? Drupal? You just need to paste that code into any BODY tags you want them to be.
@ daniil-furman You have two options to do it. 1. Edit Theme directly - Read this guide 2.Use a plugin - easy adsense is the famous plugin to do that. It dont depend on your hosting provider.