line 16 is all federal income tax withheld from w-2's and 1099's
line 18 is line 16 less line 17. since you say 16 and 18 are the same there should be nothing on line 17
on w-2's it will be the amounts in box 2
for 1099's it's box 4
You posted this question under the category of "State Tax Filing" so are you asking about a state tax return?
If so, which state?
Or did you post in the wrong subject and are actually referring to the lines on the Federal tax return?