If someone shared a Google Doc or Sheet with as "view-only", you need to make your own copy of it before you can edit it. If they wanted you to make edits to the original (collaboration), then you'd be able to edit the original. Here is how you can make your own copy of a shared document with view-only access:
1. Open the e-mail with the link to the shared document. Click on the link.
2. When the document opens, click on the FILE pull-down menu and choose MAKE A COPY.
3. Give your copy a unique name and click OK.
4. It will open a new browser tab (or window) with your copy. You can now edit it to your heart's content.
Important Note: Google automatically saves after every slight change you make. Before you close out of it, make sure it says "All Changes Saved in Drive" at the top. Although rare, I have seen people close out of it before it says this and lose their edits.
Some additional information about saving copies:
The Lunch Count spreadsheet that you copied is your own copy of it. Each time you open it and make changes, you write over the previous changes. So, if you fill it out one day, print it and give it to the Cafeteria, then open it the next day, unless you make another copy of it and save it under a different name, you will write over the previous day's. You may want that, or you may not-- it's totally up to you. If you wanted to keep a weeks worth you might make a copy and save it with the current date in the name. That way each day has a different name.