Go to google homepage and at the bottom (indicated in orange) you will see a link that says "advertising programs" cick this link
Step 2
click on the sign up now link (indicated in orange)
Step 3
fill out thr application filling in your website address in the url field and selecting personal account from the accout type (indicated in orange)
Step 4
you will have to wait untill google approves your website if it is approved you will be sent an email to tell yu that adsense has been approved and you will start noticing ads on your site like this