First you should have a blog or site.. you can use free platform like blogger or wordpress.. Set up the blog properly and giving good and unique content Then you can start sign up for adsense.. after all done wait about 1 to 3 days before your account get approve.
You first have to go to google adsense website (just search google adsense in google) and then register an account. After registering (note that you are required to submit a website for approval), login to your account and fill in the required fields. Then you just have to click on the get ads button, follow the steps and then copy paste the code generated for you into your html file. Thats all and goodluck.