If you have not set 'flutter sdk' path before. then go to File->Settings
If you have not installed plugin before then click on 'plugins' tab and install flutter, dart and kotlin plugins. These 3 plugins are essentials for flutter to run. If these are previously done you can skip these steps.
From left pane, open 'Languages and Frameworks' and click on 'flutter', then on right panel set path of flutter folder that you have downloaded in first step
It will set Dart SDK automatically, but if does not. then from left panel click on 'dart' and set 'dart-sdk' path. Your path should be similar to: ..\flutter\bin\cache\dart-sdk.