How to lock cells and sheets in Google Sheets

Right click on a cell in the range you want to lock

Hover your mouse over "View more cell actions" at the bottom of the menu and click "Protect range"

A side panel will appear. Click "Add a sheet or range"

If you just want to lock a specific group of cells, select "Range" and input the cell range in the range field

If you want to protect a whole sheet, select "Sheet"

Select the sheet you want to lock by opening up the dropdown

Click "Set permissions"

You can set it so only you can edit the restricted range/sheet, or you can set a custom permission.

To finalize, click "Done"