1-Select the cell(s)

2-Go to Format in the toolbar, Format Conditional Formatting...

3-Condition1, choose "Cell Value Is"

4-Next drop choose, "Less than"

5-Place a "0" (zero) in the next box

6-Click the "Format..." button

7-Click the "Font" Tab

8-Click the "Color" drop down

9-Click on the RED square

10-Click the "OK" button

DONE!