Allow running of scripts (must be admin)

Set-ExecutionPolicy -ExecutionPolicy RemoteSigned

Check that a file exists

if(Test-Pah file.txt) { #your code here }
if(!(Test-Path file.txt)) { #your code here }

Get the current directory of your script

$mydir = (Split-Path -Parent $MyInvocation.MyCommand.Definition)

Combine your current directory and build a new path

$mydir | Join-Path -ChildDir "subdir" | Join-Path -ChildDir somefile.txt