Have you looked at the Microsoft Virtual Academy and TechNet Virtual Labs stuff on PowerShell? In either place, search for PowerShell and you'll find a number of classes and labs. I think the one that helped me the most was the "Windows Server 2012 R2 - Windows PowerShell Fundamentals" Virtual Lab. Do all the labs you can find on ITPro.TV and TechNet that relate to PowerShell. You can't break anything there. Sooner or later you'll start feeling more comfortable.
Pretty much all of the command prompt commands and syntax work in PowerShell. Get comfortable with the Get-Help and Get-Command commands. You can use them to find and figure out how to use all the other commands. Get commands are always safe; they don't change anything. Be more careful with the other types of commands like Set and Remove.
Don't even try to memorize all the commands; there are just way to many. Learning the general syntax for powershell commands can help you figure out what a given command is likely to do even if you don't specifically know it. Practice tests will give you a good idea which groups of commands you should know for a given exam.
Hope that helped a little.