The first most important thing that you will need to know is the workspace in Photopea.  Below please find an introduction video to help you get familiar with it.  Even though it’s titled as a Photoshop tour, Photopea is the free version and exactly the same as Photoshop.   It’s a little lengthy; however, I found this guy to have the best tutorials for beginners and his voice is very soothing.  So grab a hot or cold drink and begin your training.