must know rules for blogger before blogging....

You should read the terms of service before joining, here they are: http://www.blogger.com/go/terms

There aren't any: that's the best thing about blogging (and the point of it, for many people). There are certain "accepted practices" which will get you better traffic over time, though, such as writing with personality, regular posts, producing quality writing and the usual marketing mantra of being different/interesting/offensive/fun etc.