Using Google Sheets on a desktop or laptop simply requires that you follow a link. However, if you want to be able to view and edit your spreadsheet on a tablet or mobile, you will need to go through the following steps:
Create a Google Account which you can do HERE.
Send your new Google Account email address (a.k.a Gmail) to your coach.
Download the Google Sheets app on your phone or tablet:
Open the app and sign in using your Google Account. If there is an option, set it to be signed in all the time or to remember your username and password.
After your coach has shared the document with you, you should be able to see the document in your app under the Shared with me folder:
You should now be able to view and edit your spreadsheet natively.