Getting nervous ;-}
Relax take a deep breath.... The hardware side is fully covered in the first part. I'll cover the OS piece here.
OK, I'm back...
If you can create a OS installer USB thumb drive following this guide: How to create a bootable macOS Sierra installer drive (I recommend you stay with Sierra as High Sierra has some issues with SATA based SSD's)
If you have anything on your old drive you care about you should have a backup as you won't be able to access your drive unless you get this: Universal Drive Adapter to gain access to it later.
OK, so we have the drive installed and you have the USB OS installer plugged in so all you need to do now is turn on the system and give the system some time to load the Installers OS (remember your systems USB port is slloooww!!)
Once its up you want to select from the menu Drive Utilities so you can format the SSD drive GUID partition table and Journaled volume. Once done jump to the OS installer just make sure you've selected the correct drive and go for a run, go to the gym or something else as its going to take some time.
The system will reboot a few times so don't freakout. Press the space bar as the screen will shutdown if left alone.