Hi,

Please go through the below code, I think it will helps for you

public with sharing class AttachmentUploadCon {

    public String parentId { get; set; }

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

  public PageReference upload() {

    attachment.OwnerId = UserInfo.getUserId();
    attachment.ParentId = '008521547896510'; // 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;
  }

}

<apex:page controller="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:pageBlockSection>

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

Thanks,
Raghavendra Reddy.D