In a literal sense it is simple, really:
- "Я буду идти" literally means "I will be going" and should be predominantly used when you want to talk about the process of going somewhere. For example: "I will be enjoying sunny weather when I will be going to the store". You also need to remember that "идти" means "to walk" and not "to go", so you cannot use in the context of driving, flying or using any other mode of transportation.
- "Я пойду" means "I will go" and should be used when speaking about future act of going (walking!) smewhere. To re-use the previous example, you can say "I will enjoy the weather when I will go to the store".
Having said that, the diferences are somewhat suble. Same as in English, where you can also say "I will enjoy sunny whether when I go to the store" instead of "I will be enjoying sunny weather when I will be going to the store", in Russian you can say "я получу удовольствие от солнечной погоды когда я пойду в магазин"insdead of "я буду получать удовольствие от солнечной погоды когда я буду идти в магазин". It is just in the latter case you are making an emphasis on the ongoing process, where is in the former one you are taking more about the result.
Let me know if you need more explanations.