If you haven't already, create a chart from your desired data.

Double click your chart to open the Chart editor.

Position
Auto
Legend font
Theme Default: Arial
Legend font size
 
Legend format
Text color
Auto
 

Click the "Position" dropdown and select your desired legend location.

Optionally make your legend text bold or italics.

Google Sheets is a powerful online tool to help you do quantitative analysis. Learn how to use formulas to manipulate and analyze datasets, such as Google Sheets' COUNTIF and SUMIF functions, among many others.

How to add a legend to your chart in Google Sheets