Once you have done that, you can test your web app in your browser:
<YOUR SCRIPT URL>?FN=readSheet&ID=<YOUR SPREADSHEETID>&SH=<YOUR SHEET NAME>
Looks like this:
https://script.google.com/macros/s/AKfycbyNECxxxCzY0HXXXa8W49LEJHGQ1MeuWeqRqhYmag6BMtINYkkI/exec?FN=readSheet&ID=1O1tRBxxxjs1MiFvVs0VCXXXJYTc4g_GB8wfVO3A9Es&SH=Sheet1
This should return all the data on your sheet