Okay so your ideal plan is this:
Get a 512GB SSD (or more if you can afford it) for ~$150 and replace your hard drive with it. Here you install your OS and apps, primarily - giving you near-instant boot times, apps open right away, and the whole thing feels like a brand new Mac.
Then get one of these:
https://www.amazon.com/ZXUY-Unibody...H2US/ref=sr_1_1?ie=UTF8&qid=1483152933&sr=8-1
The above part goes in place of your optical drive, allowing you to place a hard drive there instead. This is a good option to add 2TB:
https://www.amazon.com/Seagate-Barr...e=UTF8&qid=1483153014&sr=8-1&keywords=2tb+2.5
This guide will show you how to remove the current optical drive and install a "new one", in this case a huge HDD instead!
https://www.ifixit.com/Guide/MacBook+Pro+15-Inch+Unibody+Mid+2010+Optical+Drive+Replacement/3074
(Not sure if you've got a 15-inch or 13-inch, but they have guides for both).
Hardest part of the whole process for a newcomer is getting the operating system installed on the new SSD. You can download the latest Mac OS from the App Store, and there's a guide on how to create a bootable installer out of a cheapo 8GB USB stick using this app:
https://macdaddy.io/install-disk-creator/
Good luck! When the dust settles you'll have a lightning fast 512GB SSD+2TB HD setup with both speed and lots of storage.