Select any cell in your data range
Go to the Insert tab
Click Table
Check My table has headers if applicable
Click OK
Click anywhere inside the table
Go to the Table Design tab
Check Total Row
Use the drop-down in the total row cell to choose Sum, Average, Count, or another function
To add a total row without a table, click the row below your data
Enter a formula such as =SUM(A2:A10)
Press Enter