Click on the cell just to the right of the column, and just below the row you want to freeze, and then select View→Window→Freeze Panes→Freeze Panes.
So, if you want to freeze row 1 and the column A, click on B2 and select View→Window→Freeze Panes→Freeze Panes.
If you want to freeze rows 1:3 and columns A:B, click on C4 and select "Freeze Panes".
