AC: It used to be easier to dual boot systems, but with the advent of UEFI and fast start and other such Microsoft-intensive crap, it’s harder. I will make two suggestions. First, back up every scrap of data that you will regret losing–particularly family pictures, spouses have no sense of humor. Second, read the following:
It’s the most recent and best guide for what you want to do. Now read it again. And again.
Double check that your data is backed up on external media or in the cloud–DropBox is good for this.
You may not want to install Mint, but it’s really the best one for new folks. Later, you can try a different distribution. Believe it or not, the booting-from-usb method isn’t the worst thing you can do. Look into setting up a usb stick with a persistent distro (it remembers your data and changes). But start with Mint, following the reference.
The best thing I can do for you is point you at the great advice from the smarter people on this forum.
Cheers!
[Later note: subsequent comments mention two major things to remember: always install Windows first, then Linux, and using separate hard drives does indeed make things easier. Thanks, guys.]