You can set your monetization options while your video is still to be posted on YouTube.
Click the Monetization tab on YouTube
Make sure that the Usage Policy is for monetize in all countries.
Choose the ad formats you want to enable.
In the Video Manager, click the Edit button for the video you want to monetize.
Follow the same steps above.
Which ad formats should I choose?
Overlay InVideo Ads: These are the 'banners' that appear at the bottom of your video, usually around the 15-second mark.
TrueView InStream Ads: These ads are usually longer than 15 seconds, but allow a user to ignore the video after 5 seconds.
Standard Sequence Ads: These are the 15 second ads that run before your video starts. Unlike the TrueView format, these can not be ignored.
Standard in-stream ads up to 30 seconds in length: Similar to the previous format, these ads can be up to 30 seconds long. It's usually best to allow 30-second ads on longer videos only.