How to stock up?

I'm 14 weeks pregnant and a first time mom. I'm ready to start stocking up on diapers, wipes, shampoos, etc. but I have questions. I'm hoping to get enough of everything to last six months or longer. How many of each size of diapers should I stock up on? What size are babies in the longest? Should I get different brands? How many wipes and shampoos, etc. Should I also get those in different brands? Where is the cheapest places to get these things? Any help is appreciated.