Hi
A SSD is a great way of speeding up your Mac!
First of all you are going to want to install you new SSD by flooring this amazing guide!
[guide|7513|new_window=true]
Second of all you are going to want to boot into recovery mode and reinstall macOS by following along with this video
[link|https://m.youtube.com/watch?v=HCrl_4k0aqo|https://m.youtube.com/watch?v=HCrl_4k0aqo|new_window=true]
As for the version I would High Sierra as it’s the best one that your Mac can sim natively as far as I know
Hopefully this helps
Any questions please ask
Thanks:-)