I've been feeling more tired and have some drooping eyelid. When should I see the doctor?

Now. Feeling tired can be from such a large number of causes, that it would really be best to have your primary care doctor see you and do a complete check. The drooping eyelid should also be evaluated thoroughly by a doctor, as again, it could be caused by many different things.
See a doctor now. The combination of a drooping eyelid and fatigue needs to be evaluated as they are often related. An ophthalmologist can pretty easily diagnose the cause of your symptoms and initiate the appropriate therapy or refer you to a specialist who will be able to do so.
Depends. Depends as what you call "droop". If there is sagging and folding skin on your upper lids then blepharoplasty is the solution. Sometime the drooping manifest toward the end of the day when you are tired of lifting your forhead to see. Also the muscle of the eyelid ( levator) is stretched out and specific surgery to fix the musle (ptosis repair) is the solution.