I\'d like to check if a user account already exists in the system.
$SamAc = Read-Host \'What is your username?\' $User = Get-ADUser -Filter {sAMAccountName -
Simply remove the quotes around your variable:
$SamAc = Read-Host 'What is your username?'
$User = Get-ADUser -Filter {sAMAccountName -eq $SamAc}
This should work just fine.