I love Swift and iOS developmen, I trying hard to learn to it. I have started to understand Swift and Xcode, however there are so many frameworks, API's and classes to know. Which one should I start to learn first?
UIKit will be your friend in most cases; The best way to narrow the field, tho, is to make a short list of the kinds of apps you want to make. Game related APIs aren't going to be worth learning out of the blocks if you're trying to build a healthkit app.
See https://developer.apple.com/library/content/referencelibrary/GettingStarted/DevelopiOSAppsSwift/
Note the foodtracker sample/tutorial is a bit broken, so don't take it too seriously if you end up having issues, I think. Get what you can out of it and move on.
And be sure to see the iOS 10 short list here: https://developer.apple.com/ios/