How do I get the sum of all the numbers in a column?

This and all similar questions are best answered with an AWK one-liner. 

awk '{sum += $1} END {print sum}' myfile

A small bit of effort can adapt this to most similar tasks (finding the average, skipping lines with the wrong number of fields, etc.).

For more examples of using awk, see handy one-liners for awk.