<apex:pageBlockSectionItem >
              <apex:outputlabel value="Object Names :"/>
                  <apex:actionRegion >
                       <apex:selectList value="{!selectedObject}" size="1">
                                <apex:selectOptions value="{!ObjectNames}"/>
                                <apex:actionSupport event="onchange" rerender="myFields"/>
                        </apex:selectList>
                 </apex:actionRegion>
          </apex:pageBlockSectionItem>

          <apex:pageBlockSectionItem >
                  <apex:outputlabel value="Field Names :"/>
                  <apex:outputPanel id="myFields">
                    <apex:actionRegion >
                       <apex:selectList value="{!selectedField}" size="1">
                            <apex:selectOptions value="{!ObjectFields}"/>
                        </apex:selectList>
                    </apex:actionRegion>
                 </apex:outputPanel>
          </apex:pageBlockSectionItem>

      </apex:pageBlockSection>
  </apex:pageBlock>

public String selectedObject {get; set;}

public String selectedField {get; set;}

Public mappingController()
{
    selectedObject = 'account';
}

public List<SelectOption> getObjectNames()
{
    List<SelectOption> objNames = new List<SelectOption>();
    List<String> entities = new List<String>(schemaMap.keySet());
    entities.sort();
    for(String name : entities)
    {
        objNames.add(new SelectOption(name,name));
    }
    return objNames;
 }

 public List<SelectOption> getObjectFields()
 {
        Map<String, Schema.SObjectType> schemaMap = Schema.getGlobalDescribe();
        Schema.SObjectType ObjectSchema = schemaMap.get(selectedObject);
        Map<String, Schema.SObjectField> fieldMap = ObjectSchema.getDescribe().fields.getMap();
        List<SelectOption> fieldNames = new List<SelectOption>();
        for (String fieldName: fieldMap.keySet())
        {
            fieldNames.add(new SelectOption(fieldName,fieldName));
          //fieldMap.get(fieldName).getDescribe().getLabel();//It provides to get the object fields label.
        }
        return fieldNames;
  }