fasting is not starving because your body uses it's own fat stores as fuel. If the fat ran out, then the body would use it's muscle tissue as fuel... and that would constitute starving. For most people, this would not occur for a good many days, perhaps 30-40 or more.
"can i fast for 15-40 days my first ever fast?"
In my opinion, that would be too agressive for a first timer. I would start with a shorter one (even 3 days would be awesome), then look forward to the next... meanwhile learning more about it.