I'm new at Node.js and i need some practice on it, so is there any online exercises same as http://www.codecademy.com/ for node.js?
There is a node.js course at CodeSchool (the first lesson is free, $29/mo after): http://node.codeschool.com/
Just do the tutorials as you read them. Try setting up a webserver and serving a page that says "hello." I'm not sure you're going to find interactive exercises like codeacademy for something like nodejs.
Again I think your best bet is the previous SO question How do I get started with Node.js