Select the cell below the row you want to keep visible and to the right of the column you want to keep visible
Go to the View tab
Click Freeze Panes
Select Freeze Panes from the dropdown
To freeze only the top row, go to View, click Freeze Panes, then select Freeze Top Row
To freeze only the first column, go to View, click Freeze Panes, then select Freeze First Column
To unfreeze, go to View, click Freeze Panes, then select Unfreeze Panes