Select the row directly below the last row you want to freeze
Go to the View tab
Click Freeze Panes
Choose Freeze Panes from the dropdown
To freeze the top several rows, select the row below them before freezing
To freeze rows and columns together, select the cell below and to the right of the rows and columns you want frozen
To unfreeze, go to View, click Freeze Panes, then choose Unfreeze Panes