I found this article on UX collectives about typography and it seems quite helpful.
here is the low-down:
Size
The size of your fonts when designing for iOS should be no less than 11 pts to maintain legibility on the iPhone, iPad and Apple Watch. This is the minimum value of the font size, however, you will find that the ideal value lies in the range of 15–19 pt.
Weight
When using any modern font such as SF, Proxima Nova or Museo you will find they come with several weight options such as Thin, Ultralight, Medium, Semibold, Bold, Heavy and Regular. Font size and weight complement each other. It is useful to keep in mind that at 11–19 pt use the Regular weight, at20–34 pt use Medium and at 34 pt or more use Bold. You can also use the Light and Ultralight variants when the font size is very large since the large size of the font maintains legibility at these weights, however, it is important to keep in mind that larger titles in iOS 11 are generally Bolder.
Line Height
Ideally, the line height should be in the range of 120%-145% of the font size. A line height that is less than that makes the text look cramped and harder to read. When the line-height lies in the ideal range each line of text has its own space and room to breathe and everything becomes greatly legible.
Characters Per Line
Human beings read text that is more vertical faster than text that is more horizontal and has a lot of characters per line. When the line is too long the reader experiences cognitive load because he/she needs to focus on each word horizontally. This can cause reading fatigue. Ideally, there should be 45–90 characters per line to maintain a fluid reading experience.
Information Hierarchy
It is beneficial to communicate the hierarchy and interactions through your typeface. Use a bold large size font for headings, a grey color Medium weight 14–15 pt text for captions. Use reduced opacity for text that does not convey primary information.
Sans-Serif and Serif
Serif fonts have small protruding features that the sans-serif font does not have. They are both good choices based on what kind of app it is. A serif font is more commonly used in apps that engage the user with a lot of reading such as Medium, iBooks, Bloomberg and The New York Times.
Sans-serif fonts are more widely used since they are a neutral and safer choice. You will see sans-serif fonts in majority apps on the App Store.
Guides
Typography, of course, is not limited to an iOS app, the principles extend to all contexts where a human being needs to read typography.
Typography Guide
This guide can tell you more about how to combine typefaces, apostrophes, quotes, parenthesis etc
A Five Minute Guide To Better Typography
A joyful experience with animated segments that talk about every single aspect of typography in a way that will leave you with a smile on your face. Highly recommended.
Pairing Fonts
You will eventually find yourself wanting to create typographic hierarchy and contrast through the use of more than one font. This guide is really helpful in giving you a perspective that lets you pick font pairings in a much better way.
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