Enabling Unity analytics in your games is fairly simple. Point you browser to the Sign up page and follow onscreen instructions. It's free to all Unity developers!
The Unity Analytics is working on all Unity available platforms.
As soon as you done with the Unity Analytics Setup, feel free to enable this service in the plugin settings, as described here.
The Unified Analytics API as designed to match Unity Analytics. Which means, it's super easy to switch to the Unified Analytics API from Unity Analytics, and you will not miss any feature of the Unity Analytics while using Unified Analytics API.