Choose a Tool
If you have a Windows computer, do the exercise for Photoshop.
If you have a Mac computer, do the exercise for Sketch App.
Download a trial version from the website: http://www.adobe.com/products/photoshop.html
Take the free course on Udemy: Photoshop CC for Web Design Beginners
Complete the lectures in "Creating your first Web Page Design".
Learn Sketch App
Download a trial version from the website: https://www.sketchapp.com/
Take the free course on Udemy: Hands on Sketch 3 Training - Mobile App Design
Complete the lectures in "Hands-On: Duplicating an App Screen".