I am a digital artist. I am told to get external storage since I am running out of space. What is best storage? I store a lot of files!

Also I might not have an "image" of my computer & will need to have one. What the.....? Anyway will I need a disc drive for this? (I don't have one - only USB}

I have been getting the messages: "You are running out of space on C-drive making updates difficult. And my dear computer has been struggling lately! Help please!