I get headaches if I dont have a soda a day. I know, a soda a day isn't terrible, BUT, should you drink more than that, if you are trying to lose weight? Any kind of soda better than others?

well a soda a day is also bad firstly cause you want to loose weight and would even be exercising. having soda sometimes is ok but not everyday.
first of all it is not at all nutritious and secondly it harms your health. drink juice instead of soda is ma best suggestion.

What do you suggest for someone that needs the daily caffeine to avoid being really tired, or getting a massive headache?

If your loosing weight/on a diet, i suggest you to drink at least 1 bottle per day but if really possible, 1 bottle per 3-5 days.. That will not be a very big problem Well goodluck on your diet my friend

If you're looking to lose gradual weight, it's ideally to initiate yourself to cut back on junk food. That would include soda too. But you don't have to completely cut it out, just reduce the amount. So if you're drinking a six pack a day, try to reduce it by half a day. And so on.