After opening a spreadsheet, naming some columns, and putting some numbers in a column, how to I open the menu that gives me choices like sum?

Select a cell and type an = into it to open the formula editing box. The list of formulas will appear on the right side.

Three more ways:

1 - From the menu

2 - From the Toolbar:

3 - Drag a quick calculation into the cell:

If you are new to Numbers be sure to have a look at the User Guide at Help > Numbers Help in your menu, in particular the 'Enter formulas and functions' section.

SG