Tomorrow I will visit my teacher. is correct. You do not need the comma as there is no ambiguity in this sentence. We use the comma to prevent ambiguity from occurring in a sentence. For example. I'm telling you today, I will visit my teacher. (You are doing the telling today Friday the 20th that you are going to visit your teacher. This visit could take place any time in the future.) I'm telling you, today I will visit my teacher. (You are now saying that you will visit your teacher today Friday the 20th). Also if you didn't use a comma at all it could sound like you were going to visit your teacher today Friday. This is the best example I could think of off hand.

Both work. If you want to emphasize the pause after "tomorrow" then you can use a comma.

In more formal British English we were traditionally taught to use the comma as little as possible. It is often overused, particularly in situations where it is redundant, and can therefore become the source of confusion itself. I tend to use it, as you should, for lists or as I have above; to indicate the beginning and end of an extra piece of information. You are almost using it in place of brackets: ..."is often overused (particularly in situations where it is redundant) and can therefore"... I believe it is used more frequently in the US and it is certainly used more than it should be in informal written English. I would not use it in the example you have provided. In my opinion it adds nothing.

Note: I'm a US native speaker... and I'm old. I feel that both are correct. I'm afraid this is a matter of style and taste. I see that in the responses so far, everyone agrees that it is fine to omit the comma, and there is disagreement about including it--so omitting it is your safe choice. Although punctuation is used to group words and show logical structure, another way to think of it is that punctuation shows the places where a speaker would pause. A comma shows a short pause, a semicolon shows a longer pause, a period shows an even longer pause. Pauses carry meaning, so we need symbols for pauses just as we need symbols for rests in music. So, one answer is that you would or would not use a comma depending on how you mentally "hear" the sentence in your head. Generally speaking, there is a tendency for modern writers to use less punctuation than they did a century ago. When there is a choice, more punctuation feels more formal, even old-fashioned; less punctuation feels less formal and more contemporary. In your example, personally, I would use the comma if the idea of "tomorrow" were important and I wanted to put emphasis on it. But it is a matter of style, not grammar. Here are two possible examples of situations in which I personally might or might not use a comma. In each case, the punctuation corresponds to the way I would read the sentence aloud. "I'm sorry, we'll have to plan for a different day. Tomorrow I will visit my teacher." "Today, I will get my driver's license renewed. Tomorrow, I will visit my teacher."