there is a number of ways. one quick way is to:
1. select B1, then choose conditional formatting
2. window pops up...select the top option "Highlight Cell rules"
3. window pops up...select "less than"
4. in next window, simply type 0 in the text box
5. then to the right of that is a drop down menu. select the bottom option "custom format"
6. change both the FONT and the FILL to red. Hit OK.
7. Hit OK again.
8. In B1 type, =A1
9. type in your number below zero.....
8. Success!!!!!!!!!