where can i do some english grammar exercise online??

You can in fact just search for 'English Grammar Practice' on google, but I found this following website for you (I think it's pretty good!)
www.speakspeak.com/html/d3_resources_grammar_practice.htm

In addition, if you really want to improve English grammar, try to find a place where the language becomes a must-use...That would help - a lot more, and better, than just online practices ^^