Once you reach the monetization eligibility of 4K watch hours and 1K subscribers in the last 12 months, you'll be notified in the Status and features page in Creator Studio. If you're eligible, you can apply and it'll be reviewed by our specialists: https://goo.gl/aQKTKG
Noted! Have you explored our Creator Academy? Browse a huge catalog of free, online tutorials. Here's one about that specifically goes over techniques for growing your audience: https://goo.gl/jck1Hk . Hope this helps.