I am quit confused on this? I read some SEO articles that I should have to do 301 redirect so have better ranking- coz people linking your site with www and the other doesn't have. My question is which one of this- I should used in my link building? Does 301 redirect necessary?
You should stick to one form of your domain name. It doesn't matter whether you choose http://myurl.com or http://www.myurl.com. Just choose one and be consistent. Do setup 301 redirects so that you have only one specific url for each page. You may also want to look into Google's new canonical tag that helps with this issue. Plus you can set your preferred domain name in Google Webmaster Tools.
I personally always go with www. first because most people link that way. According to Yahoo, "google.com" has 5.7 million links where as "www.google.com" has 294 million links.
Amen and be consistent. Most people who link back to you will include the "www" so this is the "best" choice.
it doesnt matter which one u use. but if you find yuor site indexed for both www and non-www version of URLs, then you should do 301