Dell xps 15 7590 preparing to sell

Hi

i am repairing to sell my DELL XPS 15 7590 computer.

I wanted to ask others that may be more knowledgeable with computers about the process of preparing the laptop for sale for security reasons as well as my own data.

I mostly use google drive, adobe, and google one for storing my items.  As well as how exactly to clear the laptop.  It's on windows 10.  

Because google drive has changed so much I was also curious if anyone would have a particular recommendation on another backup?  Such as an external hard drive or anything simple for folks like me  I have a ton of pictures and data and think it may be good to have an alternative.  Any advice or help is greatly appreciated!

Megan