There are several ways.
1. Select the cells A1:C6, start typing, press Enter, type some more, repeat. It will take you to the cells in the order you want providing you've kept Direction: Down for After pressing Enter, move selection at the top of the Editing Options of the Advanced section of Excel's Options (the default)
2. Select the cells A1:C6, go into Format cells and in the Protection tab, untick the Locked checkbox. Then protect the sheet, unticking the checkbox Select Locked Cells before you click OK. (The same applies about After pressing Enter, move selection in solution 1 above)



If neither of these is good enough then come back.