How do I instruct drive to back up Mac?

[lgracey]

Hi: My drive was initially formatted and my Mac backed up at Best Buy. I now wish to back up my computer, but can’t find the instructions to do so. The drive icon
is on my desk top and appears in my finder window. Can’t figure out what to do now. Can someone please instruct me? Figure it’s a simple matter. Many thanks for your help. lgracey