The easiest way is to go to MyNU and click on Google Apps @ NU, then sign in with your Purple Pass.
Or you can go to https://www.google.com/ and click on Google Applications (looks like a waffle) and choose the App you want.
You will be asked to sign in with your g-mail account which ends with @mail.niagara.edu then click next.
A new window should pop up directing you to fill in your Purple Pass (s1234567).
Google Chrome is the preferred Web Browser for not only Google Applications but for Canvas and Self-Service as well.
You can download the web browser here for free.
Make sure your Sync is Turned On for your @mail.niagara.edu Google Account that way whenever you open a Google Application, you will be logging in with the correct account and have access to all of your passwords, bookmarks, and more.
To find out if you have Sync Turned On in Google Chrome, click the little circle icon in the top right of the web browser.