SwiftUI Prototyping would not be possible without the many resources already created by amazingly talents designers and developers. Check out, follow, buy and generally support this vital work!
The Human Interface Guidelines are your starting point for designing on Apple devices. Broadly this document explains the rationale for Apple's interfaces and how you should approach design on those platforms. The iOS platform has excellent resources for visual design including Color and Typography. An important consideration with typography is dynamic text sizes, which like Dark mode and other system level features, should be respected. Vital to all iOS app design and development, the Accessibility section is a must read especially in relation to typography.
You will eventually need Xcode to create your app prototype. The latest non-beta version can be downloaded from the Mac App Store. SwiftUI launched with some excellent tutorials that are worth checking out. This is great way to get started.
Paul Hudson runs Hacking with Swift and has provided a lot of SwiftUI content too. Basically if you google a Swift function, Paul's site will come up most times (for a very good reason).
Majid Jabrayilov has the site Swift with Majid, regular tweets amazing resources and puts out a weekly newsletter on SwiftUI (do subscribe).
Jordan Singer probably doesn't sleep considering how much code he posts on github. Follow him on twitter and see how an amazing designer inspires the rest of us.
AppCoda & book
Udemy