My phone is less than 1 year old, and just recently has started acting REALLY weird! I am assuming its a manufacturers defect, how do I go about replacing it?
If it is less than one year old and undamaged is still under Apple's warranty. Your best bet is to go to an Apple store or deal with Apple directly, In my opinion.
Yes, to Weth's point take it to an Apple Store. Here is what I would suggest first based upon my experiences with Apple:
If you've done all of that you will set yourself up for success when going to Apple.