Depends who you ask. Some people take creatine pre-workout, others take it after. The only thing that really matters is that your muscles have excess creatine in them. If you take it pre workout I would also take in some juice with sugar in it, like grape juice. Personally, I take mine in the morning, and that has worked fine for me (I lift at night).
At 16, I wouldn't expect big time gains. Your testosterone should be in full swing, so if you arent getting bigger and stronger I would doublecheck your workout program. Eating lots of good foods will do more for you than any pills, powders or other supplement.
That being said, creatine can certainly help with endurance and adding more reps and sets to each workout session. You want to make sure that you buy from a reputable company (I have heard many good things about Creapure). You will need to be drinking upwards of 1.5 gallons of water a day in order to prevent dehydration.