You need to be the best example of someone in your niche. Be the authority, but generous and kind and helpful whenever you can. Excellent customer service extends beyond what you do in your video and will go a long ways to helping you convince people to subscribe.
Think of the creators that you subscribed to on YouTube. What did they do to cause you to subscribe to them? What are things that other creators have done that caused you to not subscribe? Be aware of the things that you do and why you do it, then reverse-engineer what you learn in order to provide a better experience for your viewer.
The answer should be pretty obvious. What makes you watch/listen to the videos you watch/listen to? Make content viewers want to watch. It's that simple.
This question is quite common and the answers are all over the forums. I'll be locking this post now since there are a lot of them like this and all you need to do is read posts that show you how to grow your channel. Thank you.