Remove background for security badge

I am using a trial of Photoshop Elements 2022 with 29 days left. I know NOTHING about it and really only need it for one purpose. If I can make this work easily, I'll buy it!

I create security badges. I need to use submitted photos of various sizes, but I want to remove the background and replace with a neutral color (gray). Then I need to resize the photo to a specific pixel size (ie 300px x 400px). I searched but was unsuccessful in finding a simple way to do this. 

If someone has any suggestions, I'm all ears. Keep in mind, I have NO familiarity with PSE.