You may enable eligible videos to earn money from relevant ads after you opt in YouTube channel for monetization.
To enable ads at the time you upload the video, click the "Monetization" tab on the upload page and click the "Monetize my video" check box. Click the "Save changes" button.
To enable ads After Upload, from the "Video Manager" section of YouTube account - Locate your video - Click the "Edit" or "$" button next to YouTube video. On the "Monetization" tab of the video editor, click the "Monetize my video" check box. Select which ad formats you want to enable for YouTube video. To learn more about the different options available, click the "?" icons next to each ad format. Click the "Save changes" button.
Note: YouTube video will go through a standard review process before ads begin to appear. You may be asked to submit information proving you own all necessary commercial use rights to any or all content in YouTube video. Note that if you enable monetization at the time of upload and YouTube video is set to private or unlisted, it will remain so and without ads until you make it public.