I created application using android phonegap. I want to upload file ((i.e. when I click upload button file selection dialog box will open, then I select the file then click
Phonegap Plugins are the way to go.
There are fileuploader plugins out in the git repo that you can use.
Here's links to the iPhone and Android plugins:
iPhone File UPloader
Android File UPloader
The above answer should help you. I got also a similar problem with you, it is solved as I put the fileuploader.js inside the asset/www/ folder and give a reference to it in the index.html file.
Check out this post. It points to an example on GitHub which shows how to attach a file in PhoneGap using <input type="file">
and upload it using jQuery Mobile.