Sketch is one of the best tools you can use to design UI’s for your application. It is a vector based design tool.
Sketch is easy to use. There are many people that find Photoshop or Illustrator hard to learn. If you are a beginner you’ll find sketch much easier to learn.
It allows you to design interfaces for android, iOS and web. The good thing about Sketch is, all the assets can be generated in required dimensions as per the platform thus making designers and developers life easier.
The following article would help you in designing your app UI.