Oh my goshness, can somebody like, tell me how to change my wallpaper on my computer?!?

so like hello everyone! i am kind of in a bindness here. so like, my wallpaper on my computer is a picture of me! but i want to change it so it's a picture of me and my BEST FRIEND BFF FOREV VERONICA! i couldn't figure this out like..so i called my computer company thingy and she said "right click on your screen" so i like grabbed a marker and wrote "click" on my screen and like told her i did and she hung up on me after calling me stoopid! like that's terrible! so like how do i change it plez help me!