You should use Start-Process and its -Wait parameter if you want to force PowerShell to wait for a program to finish before it continues:
Start-Process notepad -Wait
This will cause PowerShell to suspend until you close Notepad. You should access the program with Get-Process and call its method WaitForExit() if you want to wait for programs that you did not launch yourself, or wait for a program after you launched it. The next line will wait for all running Notepad processes to exit before PowerShell continues:
Get-Process notepad | Foreach-Object { $_.WaitForExit() }
