If you are asking a teacher to help you with a question about English on a one-to-one basis (that is, you are not raising your hand in class during a particular discussion to ask a question), you really don't have to be too specific with your initial remarks. Questions like "May I ask you a question? I'm having difficulty understanding _________________" or "Can you help me with something? I don't know what _______________ means" or "Mr. (Teacher's name)? Do you have a minute? I'm having trouble with ______________" all sound like natural conversation to a native speaker. But please never hesitate to ask your teacher a question, whether or not you are concerned about phrasing it "properly" - the teacher is more than eager to help you!