How to show visual force error?

<apex:page standardController="Account">
<apex:form >
<apex:pageBlock >
<apex:pageBlockButtons >
<apex:commandButton value="Save" action="{!Save}"/>
<apex:commandButton value="Cancel" action="{!Cancel}"/>
</apex:pageBlockButtons>
<apex:pageBlockSection title="Student Info">
<apex:inputField value="{!Account.Name}"/>
<apex:inputField value="{!Account.AccountNumber}" required="True"/>
<apex:inputField value="{!Account.AnnualRevenue}" required="True"/>
<apex:inputField value="{!Account.Description}" required="True"/>
<apex:inputField value="{!Account.Site}" required="True"/>
</apex:pageBlockSection>
</apex:pageblock>
</apex:form>
</apex:page>