Backup mid 2015 Macbook Pro

I'd like to backup my mid 2015 MacBook Pro to an external storage device . It's running MacOS 10.12.6 Sierra , 2.2 GHZ Intel core i7 processor, 16GB memory, 251 GB storage . The computer has two thunderbolt ports and two usb ports. I have read that I should buy a storage device that has twice the capacity of the computer. Any suggestions, would be appreciated ,on a good storage device to buy and a

link for the procedure to do the backup .

Thank you.