signed someome else into gmail account and my gmail account was overridden

 how do I get my gmail account back as the new account for a friend has overridden my gmail account?

Your computer may be remembering the login details for your friends gmail account. Try purging your passwords in your browser.

To give you exact instructions, which browser and browser version number are you using?

Assuming you use firefox:
Edit->Preferences->Security->clear passwords

perhaps you have bookmarked "https://mail.google.com/mail/#inbox", and you checked the option to remember your friend's account.

Simply clear browser cache and cookie and try to visit gmail.

If the situation were just that your friend's password were remembered by your browser, your friend would still not be logged in automatically (instead of you) every time you visit Gmail. Automatic login in Gmail and any other web service is achieved by cookies, not by stored passwords. Therefore, mycae's advice of clearing your stored passwords will not be effective at solving your problem.

Subhajit Bandyopadhyay's advice of clearing your cookies will solve the problem. Clearing the cache is unrelated to this problem--it may save space on your machine, but doing so will make some websites load more slowly until their data are cached again.

However, none of this is necessary. All you have to do is to click your friend's email address near the upper-right corner of the Gmail page, then click Sign Out. Then, when you return to Gmail (or just click the "Sign In to Gmail" button), you'll be given the opportunity to specify your own username and password.