If you don't really mean the column headings (A, B, C, ...) but whatever you
have in row 1 of your spreadsheet, then you want "freeze panes", so click on
row 2, then Window/ Freeze Panes. If, as well as freezing row 1, you also
wanted to freeze columns A and B (for example), then click in cell C2, and
Window/ Freeze Panes. It will freeze everything above and to the left of the
active cell.