You can start by signing up google adsense program. once you're accepted, you will get access your adsense account and take your adsense ads script to be installed on your web. Make sure your website does not violate the TOS and meet their requirements. There are many tips for succeed in adsense approval in this forum.
First you sign up with google adsense. After creating adsense account you can go to adsense side and add adsense code to your site easily.
What is AdSense Google AdSense is a free, simple way for website publishers of all sizes to earn money by displaying targeted Google ads on their websites. AdSense also lets you provide Google search to your site users, while earning money by displaying Google ads on the search results pages.
You need to sigh in into Google adsense account and then you can access to your accuont. Add yuor websites there and follow the rules.