<apex:page standardController="Request__c" extensions="LocationRequestController" sidebar="false">
<apex:form id="theform">
    <apex:pagemessages id="errorMessage"/>
    <apex:pageBlock >
    <apex:pageBlockButtons location="top">
        <apex:commandButton status="statusId" value="Save" action="{!Save}" />
        <apex:commandButton status="statusId" value="Save & Create Order" action="{!SaveAndCreateOrder}" />
        </apex:pageBlockButtons>
        <apex:pageBlockSection id="Atts">
      <apex:pageBlockSectionItem >
            <apex:outputLabel for="attribute-search-status" value="User"/>
            <apex:outputPanel styleClass="requiredInput" layout="block" >
             <apex:outputPanel styleClass="requiredBlock" layout="block"/>
              <apex:selectList id="attribute-search-status" value="{!selectedUser}" required="true" size="1" style="width:180px">
                <apex:selectOptions value="{!Users}"/>
              </apex:selectList>  
              </apex:outputPanel>   
            </apex:pageBlockSectionItem>
          
       <apex:inputField value="{!ReqUser.Location__c}" required="true"/>
      
             <apex:pageBlockSectionItem >
              <apex:outputLabel for="attribute-search-status" value="Categories"/>
            <apex:outputPanel styleClass="requiredInput" layout="block" >
             <apex:outputPanel styleClass="requiredBlock" layout="block"/>
              <apex:selectList id="attribute-search-status" value="{!selectedCategory}" size="1" required="true" style="width:180px">
                <apex:selectOptions value="{!Categories}"/>
              </apex:selectList>        
              </apex:outputPanel>   
            </apex:pageBlockSectionItem>
      
            <!--<apex:actionFunction action="{!createServiceAgreement}" name="pServiceAgreement" reRender="pServAgreement" />-->
           <apex:inputField value="{!ReqUser.ServiceAgreement__c}" required="true"/>
         
            </apex:pageBlockSection>
         </apex:pageblock> 
       
   </apex:form>    
</apex:page>

I need to populate : <apex:inputField value="{!ReqUser.ServiceAgreement__c}" required="true"/> based on below fields:
Location__c and Categories
How to use <apex:actionfunction>.Pls help

           <apex:inputField value="{!ReqUser.Location__c}" required="true"/>
     
             <apex:pageBlockSectionItem >
              <apex:outputLabel for="attribute-search-status" value="Categories"/>
            <apex:outputPanel styleClass="requiredInput" layout="block" >
             <apex:outputPanel styleClass="requiredBlock" layout="block"/>
              <apex:selectList id="attribute-search-status" value="{!selectedCategory}" size="1" required="true" style="width:180px">
                <apex:selectOptions value="{!Categories}"/>
              </apex:selectList>       
              </apex:outputPanel>  
            </apex:pageBlockSectionItem>

I need to populate : <apex:inputField value="{!ReqUser.ServiceAgreement__c}" required="true"/> based on below fields.
How to use <apex:actionfunction>.Pls help