VBA to show error message

Hi,

I am trying to build a tool in which user enters multiple data within a range A2:L100
All users must enter a comment in the column I. Without entering it users are not allowed to move on to next row. If user start from A2, B2, .... H2, he must enter a comment in I2 aswell and only then he should go to A3. If the user skips I2 and tries to enter values in A3, a warning should pop-up. I tried to my level best but couldn't succeed. Looking for your help.

Rakul.