
Uhh. The same way you use excel formulas...? It's really not that hard, it's just spreadsheets. For some general help from google see
https://support.google.com/docs/answer/46977?hl=en, and the list of functions they support is
https://support.google.com/docs/table/25273?hl=en (Keep in mind, the functions google docs & excel support differ. There are similarities, but this is not guaranteed).
So, quick example, say you have a spreadsheet that has a single column (A), and the rows 1-5 contains numbers (see image), and then you want to calculate the sum of all the numbers in that row. You go to the list of functions, search for sum, and then you find "SUM(value1, value2)", so what you do is you click on a new cell, say B1, and at the top next to Fx you type in =SUM(A:A), and the cell B1 will now contain the sum of all items on row A. If you only want to sum a subset of items from row A (say, only the first 3), you'd do =SUM(A1:A3). Keep in mind that after you type =SUM(, you can click on a cell to have its value prefilled into the function box.
@previous (D)
I also discovered that Google Drive has a button for these exact formulas too. I figured most of it out, but I didn't know about =SUM(A:A) for just the column.