I agree with the others that "I'm going back" is the correct choice. The only way that you can have the combination "I'm going to back" is in the sentence: "I'm going to back you up", which means that you're going to support someone. This support can be either physical (to help someone do some manual labor) or emotional (to support someone's opinion for instance).

"Go back" means return. We don't use "to" in this phrase. "Go to the back" means an area in a specific place or room. The back of the house (farthest from the street), or the back of the cinema (farthest from the screen).

""I'm going back" is correct. For example: "I'm going back to the store to buy more beer." This is the verb go in the present progressive tense, showing an action in progress or about to be in progress. "I'm going to back" is not correct. The "to" does not belong in the sentence. However, you could say "I'm going to go back." For example, "I'm going to go back to the store." In this sentence, the word "to" is part of the infinitive verb, "to go." Here, "going" means that you will soon be doing something -- in this case, you will soon go to the store.

I'M GOING BACK