Listen to Lily Allen and sing along.

Practice, practice and practice. I suggest to read the book "Get Rid of your Accent" by Linda James and Olga Smith.

why do you want to have a British accent,l think American accent is better

You need to actually sit down with either a British person who has a clean accent, or someone with proper experience (anything from a speech therapist to a professional actor would be fine), to get the accent right. I really don't recommend using a book. You can't learn an accent in a vacuum, and you need proper training (ie. someone else listening to you) to get it right. There are a few pointers I give students for pronunciation and placement, which will clean up thick first-language accents and make the student sound relaxed and natural.

If you want a British accent look at http://www.usingenglish.com/links/Listening/ http://www.usingenglish.com/links/Sp...Pronunciation/