To run the script analyzer against a single script file execute:
Invoke-ScriptAnalyzer -Path myscript.ps1
This will analyze your script against every built-in rule. If your script is sufficiently large that could result in a lot of warnings and/or errors.
To run the script analyzer against a whole directory, specify the folder containing the script, module and DSC files you want analyzed. Specify the Recurse parameter if you also want sub-directories searched for files to analyze.
Invoke-ScriptAnalyzer -Path . -Recurse