Hi anvesh,

Below is the sample Visualforce Page as per your requirement.

Visualforce Page:

<apex:page  tabstyle="Account">
<script>
    var newWin=null;
    function openLookupPopup(name, id)
    {
        var url="/apex/YourcustomLookuppage?namefield=" + name + "&idfield=" + id;
        newWin=window.open(url, 'Popup','height=500,width=600,left=100,top=100,resizable=no,scrollbars=yes,toolbar=no,status=no');
        if (window.focus) 
        {
            newWin.focus();
        }
            
        return false;
    }  
</script>                  
  <apex:form >
    <apex:pageBlock title="Lookup">
      <apex:pageBlockSection columns="1">
        <apex:pageBlockSectionitem >
          <apex:outputLabel value="Account"/>
          <apex:outputPanel >
            <apex:inputHidden id="targetId" />
            <apex:inputText size="40"  id="targetName" onFocus="this.blur()" disabled="false"/>
            <img src="/s.gif" onclick="openLookupPopup('{!$Component.targetName}', '{!$Component.targetId}'); return false" class="lookupIcon" onmouseout="this.className='lookupIcon';" alt="Account Name Lookup (New Window)" onmouseover="this.className='lookupIconOn';"/>           
          </apex:outputPanel>
        </apex:pageBlockSectionitem>
      </apex:pageBlockSection>
    
    </apex:pageBlock>
  </apex:form>  
 
</apex:page>

Important :
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Thank You,
Hitesh Patel
SFDC Certified Developer & Administrator
My Blog:- http://mrjavascript.blogspot.in/