oracle: not a single-group group function

hi guys

I do

select id, to_char(SUM(salary),'999,999,999,999') as salary from customer .

customer table has
id salary


I get error
ORA-00937: not a single-group group function
00937. 00000 -  "not a single-group group function"
*Cause:    
*Action:

any idea? Am i missing GROUP BY clause ? not sure if i need one.

thanks