First, you need to begin a whole new spreadsheet. Then add the initial data that is needed.
After that, select the cell that you wish to AutoFill. Move the cursor to the bottom right corner of the cell. It'll turn into a solid cross.
Note how Excel fills the series of months for you automatically. Now, drag the cursor across the cells to as many as you need, whatever amount that is.
Glad I could help, and have a good day!