1) Download this file from any one of the following link.
2) Extract it to a new folder, then navigate to this path "mimikatz_trunk-->alpha-->win32 or x64" (Choose your bit)
3) Select that "mimikatz.exe",then right click on it and click "Run as Administrator" (You must run that file with Administrator Rights)
4) Type "privilege::debug" without quotes and press enter,you will get some command as Privilege '20' OK
5) Then type "sekurlsa::logonpasswords" and press enter.Now you will get your system usernames and passwords in plain text.