Much like other modern programming languages (for example Kotlin or Swift), Dart has a free playground which you can use to get familiar with the syntax and special features. It's called DartPad and you can find it at https://dartpad.dev/ .
Dart also has most of the features you would expect if you're coming from Kotlin, Swift - or even JavaSrcript for that matter -, so if you have a past with those languages, you can catch on quickly.
This Dart overview aims at giving you the basic concepts of the language. It covers most language features used in the Flutter example projects.
For more detailed information and deeper descriptions, please visit the official tour of the Dart language.
Next up, you'll learn about Dart's type system and type inference.