I would like to know how I can authenticate the username and password for gmail. Please help me out!

Question

I am making an application that basically let the user log into its gmail account and send email. My user interface will contain username textbox and password textbox, and log in button. If the user type in the correct username and password, it will take the user to another user interface where the user can send email to another gmail user. What I want to know is, on my first user interface, how can I authenticate the username(gmail username) and password that the user type in? I've been looking over the net how to do it. I could not find it unfortunately. Please help me out. Thanks in advance.

Saing