iTunes Connect provides a portal for managing your apps and items related therein.
The following video will guide you through creating a new app on iTunes Connect and prepare you for uploading that app.
Build and submit your app to iTunes Connect via Xcode.
Screenshot sizes for portait iPhone app previews:
|iPhone Type||Size||Pixel Width||Pixel Height|
|iPhone 4/4s||3.5 inch||640||960|
|iPhone 5/5s||4 inch||640||1136|
|iPhone 6||4.7 inch||750||1334|
|iPhone 6 Plus||5.5 inch||1242||2208|
Managing your app with iTunes Connect https://developer.apple.com/library/ios/documentation/IDEs/Conceptual/AppDistributionGuide/UsingiTunesConnect/UsingiTunesConnect.html