Hey Patty37!
How big is your first HDD that has everything on it? Does it have a separate partition for the OS or is the whole drive one partition?
If it has a separate partition for the OS it would be easier. Then you could simply clone that partition onto the new SSD. Please note, that it is always better to do a fresh install rather than cloning! If there are setting, accounts, paid software etc., that you need to keep, then clone. But first clean the HDD's OS partition from temps, broken shortcuts, junks, unnecessary files etc., defrag it and ran an antivirus and malware software. Then power down the PC and plug in the SSD and use a cloning software to clone the OS partition onto the SSD.
Then restart, go into BIOS and set to load from the SSD, make sure everything is alright and see if you could access your personal files on the old HDD. If everything is ok, power down the PC again, connect the new WD Green hard drive and again load the SSD OS, partition and format the new HDD (not the old one!!) and move your personal files across the hard drives.
After you move everything make sure everything is in order again: that SSD OS works, that your new drive has all of your data and is accessible etc. Then power down, disconnect the old HDD and load your PC with your new SSD and new HDD.
If you'd like, you could either reformat the old drive to use as additional storage or just keep the drive somewhere with the data just in case. You could at least reformat the old OS partition.
It's best that you have OS and all applications and software onto the SSD and load them from there. Everything additional as files should go to the HDD. Besides, the WD Green is designed as a storage drive only – for archiving and back ups, so it's not that suitable for loading programs off it.
Hope this helps! Always have a back up first!
Good luck! :)
CK_WD