Computer advise!!!! Gb USB?? ?

Okay so I dont know much about computers but I need advise. My computers "memory" is super low I only have about 3.13 gb free out of I think 90-something gb free on my c drive and my d drive is low too... Should I buy an external hard drive (USB)?? Or just get a regular USB of 16gb?? I have a lot of music and movies on my comPuter so I was thinking that the external USB would be better. If it is should I get the wb??