How to handle file upload element in selenium webdriver?

I am writing a script in selenium using webdriver and junit to upload files on server. But unable to do it because i don't know how to handle file explorer window of Windows(OS) shows on the click of choose file button of file upload element. I have tried it by enter the file path directly in the text box and then click upload button.But still it shows please choose a file to upload. Pleas help.Its Urgent! Thanks.