How can I remove my membership from the twitter. I want to start all over again with new username?

Sign in twitter, if not already

Click http://twitter.com/settings/account

Scroll down

Click on Deactivate my account > confirm

To avail all twitter features you should have an email address attached to twitter account, this doesn't need necessarily to be a yahoo email address. It can be any email address. But make sure that email address is not already attached to a twitter account.

To be on safe side before deleting your twitter account, change its email address to any arbitrary email address. Then delete your twitter. It will keep your email address free i.e. can be used in any other twitter account.

http://twittercollection.blogspot.com/2010/08/twit...

You probably would need to change the email on the acct you have now to a different email address & then register a new acct with the the new user name & then reuse the email address you were using before.