I would suggest to start learning PowerShell and PowerCLI at the same time.
What I find a good approach is to set yourself an easy target to start with.
For example, generate a report with information from your environment.
This can be for one resource (HostSystem, VirtualMachine, Datastore...) or a combination of these.
Doing practical things is imho the most usefull way to learn, and remember, something.
There are a number of PowerShell and PowerCLI books available.
Most of these are quite good, but note that the books do not always address the same audience (beginner, intermediate, expert)
There are a lot of sites available that link to learning resources.