How to use <apex:inputFile>  in Visualforce Page?

<apex:inputFile>:
A component that creates an input field to upload a file.
Note: The maximum file size that can be uploaded via Visualforce is 10 MB.

This tag supports following attributes:

Attribute	Description
accept	Comma-delimited set of content types. This list can be used by the browser to limit the set of file options that is made available for selection. If not specified, no content type list will be sent and all file types will be accessible.
accessKey	The keyboard access key that puts the component in focus.
alt	An alternate text description of the component.
contentType	String property that stores the uploaded file's content type.
dir	The direction in which the generated HTML component should be read. Possible values include "RTL" (right to left) or "LTR" (left to right).
disabled	A Boolean value that specifies whether this component should be displayed in a disabled state. If set to true, the component appears disabled. If not specified, this value defaults to false.
fileName	String property that stores the uploaded file's name.
fileSize	Integer property that stores the uploaded file's size.
id	An identifier that allows the component to be referenced by other components in the page.
lang	The base language for the generated HTML output, for example, "en" or "en-US". For more information, see the W3C specification on this attribute: http://www.w3.org/TR/REC-html40/struct/dirlang.html
onblur	The JavaScript invoked if the onblur event occurs that is, if the focus moves off of the component.
onchange	The JavaScript invoked if the onchange event occurs that is, if the user changes the content of the component field.
onclick	The JavaScript invoked if the onclick event occurs that is, if the user clicks the component.
ondblclick	The JavaScript invoked if the ondblclick event occurs that is, if the user clicks the component twice.
onfocus	The JavaScript invoked if the onfocus event occurs that is, if the focus is on the component.
onkeydown	The JavaScript invoked if the onkeydown event occurs that is, if the user presses a keyboard key.
onkeypress	The JavaScript invoked if the onkeypress event occurs that is, if the user presses or holds down a keyboard key.
onkeyup	The JavaScript invoked if the onkeyup event occurs that is, if the user releases a keyboard key.
onmousedown	The JavaScript invoked if the onmousedown event occurs that is, if the user clicks a mouse button.
onmousemove	The JavaScript invoked if the onmousemove event occurs that is, if the user moves the mouse pointer.
onmouseout	The JavaScript invoked if the onmouseout event occurs that is, if the user moves the mouse pointer away from the component.
onmouseover	The JavaScript invoked if the onmouseover event occurs that is, if the user moves the mouse pointer over the component.
rendered	A Boolean value that specifies whether the component is rendered on the page. If not specified, this value defaults to true.
required	A Boolean value that specifies whether this component is a required field. If set to true, the user must specify a value for this component. If not selected, this value defaults to false.
size	Size of the file selection box to be displayed.
style	The style used to display the component, used primarily for adding inline CSS styles.
styleclass	The style class used to display the component, used primarily to designate which CSS styles are applied when using an external CSS stylesheet.
tabindex	The order in which this component is selected compared to other page components when a user presses the Tab key repeatedly. This value must be an integer between 0 and 32767, with component 0 being the first component that is selected when a user presses the Tab key.
title	The text displayed next to the component when the mouse hovers over it.
value	A merge field that references the controller class variable that is associated with this component. For example, if the name of the associated variable in the controller class is myInputFile, use value="#{myInputFile}" to reference the variable.

Visualforce Example: 

<apex:page standardController="Document" extensions="documentExt">

    <apex:messages />

    <apex:form id="theForm">

      <apex:pageBlock >

          <apex:pageBlockSection >

            <apex:inputFile value="{!document.body}"  filename="{!document.name}"/>

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

          </apex:pageBlockSection>

       </apex:pageBlock>

    </apex:form>

</apex:page>

Controller Class:

public class documentExt {

    public documentExt(ApexPages.StandardController controller) {

        Document d = (Document) controller.getRecord();

        d.folderid = UserInfo.getUserId(); //this puts it in My Personal Documents
    }                 

}