You should start making good videos, then if your are lucky if any of your video or videos is viewed by large number of audiences. And if it gains a good amount of views within a decent amount of time period, Youtube will automatically send you a message in your youtube account as well through E-mail which you used to sign up with youtube. The message will generally say that your particular video is eligilbe for monetization(revenue sharing) and it will give you option to learn more. You should then apply for google adsense and the follow the steps. Monetize your video(s) that are eligible for revenue sharing. If you You are ready to go....
This is on my own knowldge and personal experience !
Check out this video only for monetization process.