Google Sheets is Google's spreadsheet app. It is definitely not as feature-rich as Excel but it is free, has a huge library of add-ons that extend the functionality, and is accessible anywhere. With integration with other Google services (such as Google Forms) it is even better than Excel for certain applications.
Using Sheets
The basic functionality of Sheets is much the same as Excel. You can use many of the same formulas and shortcuts. There are also a variety of templates to choose from so it is very easy to get started with sheets and you don't need to know how to use every feature to accomplish something.