If you like a video you're watching, you can either subscribe right from under the video by hitting the "subscribe" button, or you can check out the channel first and subscribe there.
We tap the channel icon to head there before subscribing.
Once you're on a channel, you can browse the home page, videos, playlists, channels, and about page.
Once you're ready to subscribe, you can do so by heading back to the home page and tapping the subscribe button.
Subscribing reveals a new button with a bell icon. Tapping on this new button lets you get notifications about new videos from the channel you just subscribed to.