To sum all the contents of, say, Column A, write =SUM(A)
Be careful because if your sum expression is in the same column as the data, it must be in a Header Row or Footer Row, else you will get a recursion error message.
The Numbers User Guide is a good read and explains this business quite well. You can download a copy from the Help menu, along with another useful guide, the iWork Formulas and Functions User Guide. The first you should read cover to cover. The second is a reference volume that can be consulted for specific questions on Functions such as SUM.
The formula you're looking for is =SUM(B) (for the sum of the numbers in the non-header/footer cells of column B). See the red arrow, indicating this formula and the cell containing it (C2)
See also the green arrow pointing to several basic 'instant' calculations that are done using any set of cells that is selected. Results shown are for the selected cells B2:B10.
May I suggest that you will find it uiseful to download and read the Numbers '09 User Guide, available through the Help menu in Numbers. The guide contains descriptions of several of the features of Numbers, and will provide you with many of the basics concerning Numbers and spreadsheets in general.
In the same Help menu, you'll also find the iWork Formulas and Functions User Guide. This guide is a useful reference providing descriptions and examples for all 256 functions supported by Numbers '09.