+1 for the tutorial link that @JNK supplied
I'm adding an answer here that has the actual steps, that have been taken from the tutorial. You never know when one of Microsoft's links stop working.
How to Apply Conditional Formatting
To apply conditional formatting to one or more cells,
- Highlight your cell or range of cells.
- From the Format menu, select Conditional Formatting…
- In the Conditional formatting dialog, select Cell Value Is in the
first drop-down box.
- Select a cell evaluator from the second drop down box. Your dialog
may change and add another field.
- Enter your values in the remaining text boxes.
- Click the Format… button.
- Choose your format options from the Font, Border and Patterns tabs.
- Click OK.
- Click OK or add another condition.