Hi Reddy,

Are you using Vf page for Opportunity? If you are using Vf page for opportunity then you can copy below code & paste in your org.
If you are using for another object then change object name in Vf page (from here: standardController="Opportunity"). After that you can use it according your requirement which object you want to do.

Folllow the below steps:
1.) Copy Vf page & Class Extensions and paste in your org.
2.) Use for your require object & Put VF page on the pageLayout.
3.) It will show on detail page.
4.) Upload your attachment in Vf page section.
5.) It will show in Note & Attachment related list.

Vf Page:

<apex:page standardController="Opportunity" extensions="AttachmentUploadController" >  
  <apex:sectionHeader title="Visualforce Example" subtitle="Attachment Upload Example"/>

  <apex:form enctype="multipart/form-data">
    <apex:pageMessages />
    <apex:pageBlock title="Upload a Attachment">

      <apex:pageBlockButtons >
        <apex:commandButton action="{!upload}" value="Save"/>
      </apex:pageBlockButtons>

      <apex:pageBlockSection showHeader="false" columns="2" id="block1">

        <apex:pageBlockSectionItem >
          <apex:outputLabel value="File Name" for="fileName"/>
          <apex:inputText value="{!attachment.name}" id="fileName"/>
        </apex:pageBlockSectionItem>

        <apex:pageBlockSectionItem >
          <apex:outputLabel value="File" for="file"/>
          <apex:inputFile value="{!attachment.body}" filename="{!attachment.name}" id="file"/>
        </apex:pageBlockSectionItem>

        <apex:pageBlockSectionItem >
          <apex:outputLabel value="Description" for="description"/>
          <apex:inputTextarea value="{!attachment.description}" id="description"/>
        </apex:pageBlockSectionItem>

      </apex:pageBlockSection>

    </apex:pageBlock>
  </apex:form>
</apex:page>

Class Extension:

public with sharing class AttachmentUploadController {

   public string opp;

     public AttachmentUploadController(ApexPages.StandardController controller) {
            opp = controller.getRecord().id;
 
     }

     public Attachment attachment {
       get {
           if (attachment == null)
               attachment = new Attachment();
               return attachment;
           }
      set;
      }
  
 
     public PageReference upload() {

      attachment.OwnerId = UserInfo.getUserId();
      attachment.ParentId = opp; // the record the file is attached to
      attachment.IsPrivate = true;

     try {
          insert attachment;
         } 
         
         catch (DMLException e){
      
         ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR,'Error uploading attachment'));
         return null;
         } 
         
         finally {
         attachment = new Attachment(); 
         }

         ApexPages.addMessage(new ApexPages.message(ApexPages.severity.INFO,'Attachment uploaded successfully'));
         return null;
     }

}

Please use it. It will work for you.

If you have any issue please let me know.

Thanks,
Arvind Kumar