Saving PNG design <3
So after you've made a design in photoshop,you want to go over to your layers, right click on your top layer, & click "merge visible" or "flatten image", this makes all of your layers become ONE. you need to do this before ever saving a PNG, every time. see below

Now that you've done that, go to File at the top > save as. Now name your file whatever you want, & click on the scroll bar beside "Save as type" & choose PNG from that scroll bar (see below)

click save & you're done!