You can apply for monetization at any time. To be reviewed, all channels need at least 4,000 watch hours in the previous 12 months and 1,000 subscribers. This requirement allows us to properly evaluate new channels and helps protect the creator community.
After the Sign in to your YouTube account, click on the BETA section. Then click on the channels tab from the left side. Then on the right side you will get Monetization tab as shown in the image. Even the below image shows the terms & conditions associated with the monetization of your YouTube videos. Please go through those carefully and enable the option.