Soooo... how to get started?

Well, this is very new and we're a little lean on the docs at the moment. Bear with us
We've focused on making this easy for a non-programmer, and consequently the script API has a little catching up to do.

So, from the editor:

Step 1: From the main menu, select Cinemachine/Create Virtual Camera

Step 2: Drag the game object you want the virtual camera to look at onto its LookAt field in the inspector. The camera
will now look at that object. Drag the virtual camera or the object around, and see. Change the virtual camera's "Aim" method to "Composer" and you will get a bunch of interesting options.

Step 3: If you want the camera to move in space (instead of just changing its orientation), drag a game object onto the virtual camera's "Follow" field. The virtual camera's position will now be locked to that game object. You can only move the virtual camera by moving the object that it follows (try it and see). Change the virtual camera's "Body" method to "Transposer" and you get another bunch of interesting options.

That's the general idea.

 

Cinemachine timeline lets you blend between Cinemachine virtual cameras. Blends are not fades - they are more like a smooth camera movement from one position to another.

Cinemachine Virtual cameras (aka vcams) control what the Unity camera does. They are called Virtual cameras because they are not cameras. They are more akin to cameramen than to cameras. When a vcam is active, it is animating the Unity camera. Blending from one vcam to another will animate the Unity camera from one procedural state to another.

The timeline can be used to control which vcams are active when. If your game camera is implemented using Cinemachine (which is the recommended strategy), then the timeline can be used to smoothly blend to and from cutscenes. If no vcam is active, then the Unity camera will just sit where Cinemachine left it.

Drag a Cinemachine-enabled Unity camera onto the timeline, select "Create Cinemachine Track" from the popup. Then, right-click on the track to create Cinemachine Shot Clips. Each of these will control a vcam. If you overlap them, they will blend.

 

First thing to know: vcams are a way to animate the Unity Camera. To do this, the Unity camera must have a component on it called the CinemachineBrain. This is what takes the instructions from the vcams and animates the camera. A brain is automatically created on the main camera when you create your first vcam from the Cinemachine menu. However, you can add brains to other cameras by adding a CinemachineBrain component to them manually.

Second thing: to get the cinemachine option on the timeline track creation, you must drag a unity cam that already has a brain (you can tell because it gets a little CM icon in the scene tree). It's the presence of the brain that gives you the Cinemachine option.

So the steps are:
1. create a few vcams
2. create a timeline
3. drag the main camera (it now has a brain) onto the timeline, select "create cinemachine track"
4. right-click in the shot area of the track and select "create cinemachine shot clip"
5. click on the new clip to get an inspector for it
6. populate the Virtual Camera field with one of the vcams from the scene
7. repeat for additional shots and tracks

tip: it's a good idea when using timeline to lock it (click the little lock icon on the upper right) otherwise you have to keep finding it again when you click elsewhere.