CaseComment is separate object in salesforce. So I suggest you use either standard controller extension or custom controller

CaseComment is separate object in salesforce. So I suggest you use either standard controller extension or custom controller.

But If you want or have to use only standard controller here is some code:

<apex:page standardController="Case">
   <apex:form>
       <apex:repeat value="{!Case.CaseComments}" var = "cc">
           <apex:inputField value="{!cc.CommentBody}" label="Internal Comments" />
       </apex:repeat>
   </apex:form>
</apex:page>