File Upload Controller:

public with sharing class FileUploadController {

  public Document document {
    get {
      if (document == null)
        document = new Document();
      return document;
    }
    set;
  }

  public PageReference upload() {

    document.AuthorId = UserInfo.getUserId();//Current User Id
    document.FolderId = UserInfo.getUserId();//Add Folder Id

    try {
      insert document;
    } catch (DMLException e) {
      ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR,'Error uploading file'));
      return null;
    } finally {
      document.body = null;//Clear the View State
      document = new Document();
    }

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

File Upload Visualforce Page:

<apex:page controller="FileUploadController">  
  <apex:sectionHeader title="File Upload" subtitle="File Upload Example"/>

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

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

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

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

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

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

        <apex:pageBlockSectionItem >
          <apex:outputLabel value="Keywords" for="keywords"/>
          <apex:inputText value="{!document.keywords}" id="keywords"/>
        </apex:pageBlockSectionItem>
      </apex:pageBlockSection>
    </apex:pageBlock>
  </apex:form>
</apex:page>  

Output: