How do I change the material on a mesh?

Hello. Brand new to UE4. Whenever I add my own custom material to a mesh it ends up like this, with a white background of the old texture. [Pic Below] Is there anyway I can fix this?

I’m not an artist, but basically you have to erase the background of your material in your Art program before
you save it and import it into the game. Best one for me has always be png format. I worked this out ages
ago with Adobe photoshop. I am not sure if all the cheaper programs do it as well or easily. Alot programs start your image with a blank background which ends up as a white layer in the game. So using a background eraser tool choice in your image program, remove the background then resave it and see how that comes out.