I want to email my English teacher and ask him what what should I do to get better grades in English.
But I don't know to say it in a polite way,
Can somebody please tell what should I write on the email
Well I would probably talk to him in person rather than send an email (unless he's specified for you to email him if you have any questions). I don't think you really need to send a very formal letter though. This person knows who you are so just ask the question, and also say your name so he knows who you are. Although as he is an English teacher, he'd probably appreciate that you use proper spelling and grammar
It's really not a big deal. Your friends shouldn't care about it. There's nothing wrong with asking teachers for help