If you put 1 in cell A1
In cell A2 put =A1+1
Drag this formula down to cell A3 and you get 3
and so on.
If you just want to fill a series of cells put 1 in Cell A1
Put 2 in cell A2
Select both cells and drag them down the column, (use the little block in the bottom right corner of cell A2, left-click with mouse and hold down the key and drag)
and the column will fill with 3, 4, 5 and so on.
Hope one of these works for you. If not ask again.