You'll need to create two rules, unless you want one of the two formats to be the default formatting of the cells.
Select the cells you want to format.
On the Home tab of the ribbon, click Conditional Formatting > New Rule...
Select 'Format only cells that contain'.
Leave the first drop-down set to 'Cell Value'.
Select 'less than' from the second drop-down.
In the box next to it, enter the formula ="M"
Click Format...
Specify the desired formatting, for example a specific font color and/or fill color.
Click OK, then click OK again.
Repeat the above steps, but with 'greater than or equal to' instead of 'less than', and with different formatting.
It should work for entries with more than one letter too:
See the attached sample workbook.