<apex:page standardController="Opportunity" extensions="opportunityController">
    <apex:form >
       
        <apex:selectList value="{!selectedStage }" size="1">
            <apex:selectOptions value="{!PLValues}"  />
            <apex:actionSupport event="onchange" reRender="pb"/>
        </apex:selectList>
    
    <apex:outputText value="{!selectedStage}"></apex:outputText>
    
    
   <apex:pageBlock title="Opportunity details" id="pb">
      <apex:outputText value="{!selectedStage}"></apex:outputText>

        <apex:pageBlockSection id="frstpb" rendered="{!IF(selectedStage =='Qualification',true,false)}">
            <apex:inputfield value="{!Opportunity.Name}"/>
            <apex:inputfield value="{!Opportunity.StageName}"/>
            <apex:inputfield value="{!Opportunity.CloseDate}"/>
        </apex:pageBlockSection>
     
       
        </apex:pageBlock> 
        
        </apex:form>
</apex:page>

public class opportunityController{
    Opportunity record;
    
    public opportunityController(ApexPages.StandardController controller) {
        record = (Opportunity ) controller.getRecord();
    }
    
    
    public string selectedStage {get; set;}
    public List<SelectOption> getPLValues() {
        List<SelectOption> plValues = new List<SelectOption>();
        
        Schema.DescribeFieldResult dfr = Opportunity.StageName.getDescribe();
        for (Schema.PicklistEntry ple : dfr.getPicklistValues()) {
            plValues.add(new SelectOption(ple.getValue(), ple.getLabel()));
            if (ple.isDefaultValue()) {
                selectedStage = ple.getValue();
            }
        }
        
        return plValues;
    }
}