Here in below example, I’ve a requirement on selection of “Category” picklist value “Product”, “Detail” inputfield will enable with required mark else will disable.

<apex:page standardController="Campaign" showHeader="true">    
    <apex:form >
        <apex:pageBlock>
            <apex:pageBlockSection title="Information" id="pb1">
                
                <apex:pageBlockSectionItem>
                    <apex:actionRegion>  
                        <apex:inputField label="{!$ObjectType.Campaign.fields.Category__c.Label}" value="{!Campaign.Category__c}">
                                <apex:actionSupport event="onchange" rerender="pb1" />
						</apex:inputField>
                    </apex:actionRegion>
                </apex:pageBlockSectionItem>
                
                <apex:pageBlockSectionItem>
                    <apex:outputLabel value="{!$ObjectType.Campaign.fields.Detail__c.label}" for="wurl" rendered="{!Campaign.Category__c=='Product'}"/> 
                    <apex:inputField id="wurl" value="{!Campaign.Detail__c}"  rendered="{!Campaign.Category__c=='Product'}"   required="{!Campaign.Category__c=='Product'}" />
                </apex:pageBlockSectionItem>
				
            </apex:pageBlockSection>
        </apex:pageBlock>
    </apex:form>
</apex:page>