My mid-2010 MacBook Pro's hard drive is dead -- How do I choose an SSD to replace it?

I have a mid-2010 15-inch MacBook Pro with an i7 processor that recently died on me. I went into the Apple Store, and as it turns out, I need a new hard drive. Since I'm out of warranty, I thought I'd just buy one and do it myself.

I was talking to a friend, and he mentioned that an acquaintance of his put a smaller SSD in where his original hard drive was, and then put a second hard drive in where his optical drive was. That way he got the speed from the smaller SSD by installing all apps there, but then got massive storage space by installing a couple TB normal hard drive where the optical drive was.

How do I go about doing this? How do I actually choose which hard drive I want to put in? Like how do I know which ones will both fit and function inside of my computer properly? And how do I choose which one will get me the most oomph out of my computer?

Thank you for your help!