I''m writing in a loss on line 13 of 1040. Do I put it in with a negative sign? If the answer is no then how do I put it in that it won't be included in my total income?

If it is a loss, then put () around the numbers. This is how to record negative number in finance format. For example, a $123.45 loss will be (123.45) on this line.
And then at line 22, when doing the summation, you can take this amount out.

Anthony Bu
IRS VITA, SPEC area 3

What Anthony says about parentheses is true.

However, you can also list it with a negative sign with no problems. That's how the TaxAct software lists negative figures on its computer-generated tax forms.