Error message on field Validation BSP Page

Hi Experts,

I have a code on BSP Page like this :

<htmlb:label text = "*<%= otr(BBP_SUS_UI/FIRSTNAME) %>"

for = "FIRSTNAME" />

</td>

<td>

<htmlb:inputField id = "FIRSTNAME"

width = "250"

disabled = "<%= disable_change %>"

doValidate = "true"

value = "//co_model/firstname" />

</td>

</tr>

<tr>

<td>

My requirement is to create an ERROR message, there is any entry in this field for NON CHARACTER type, when I will press SAVE Button.

Thanks in advance,

Study SAP