I agree with Adam and Greg. Both sentences are fine. By the way, there is no difference between British and American English with regard to future tenses. Any native speaker might say: "I'm going to go fishing tomorrow" . This is generally pronounced, but not usually written, as "I'm gonna go fishing tomorrow". [ And no, "I'm will go fishing tomorrow" is not right].

Both are correct but you don't necessarily need the "to go" in either. "I'm going fishing tomorrow" --> is just the general expression to indicate your plan. "I'm going TO GO to the supermarket tomorrow" --> shows a sort of extra determination to your plan. EDIT: I would like to say that "going to" just indicates how certain you are of doing a plan (in this case 100% sure) and can be used to express any time in the future, from being in an hour to in 10 years" You can also use the more colloquial expression "I'm gonna" to express the exact same thing but in a more casual way.

Hello, No, It is not right. I think American English is quite different from British English. Here in the United States, going to, is used for the near future, and tomorrow is not considered in the near future. So you either have to say: I'm gonna go fishing right now or I'm gonna go fishing in a few hours. Or if you want to use the adverb "Tomorrow", You gotta say: I'm will go fishing tomorrow.

Both sentences are good.