Photos App Clear-out
You should start your new year with an empty Apple Photos App, as otherwise you are likely to have big problems with storage in the middle of the year when it will be harder to fix.
The easiest way to do this is to:
Connect an external hard drive
Make sure you have quit the iPhoto/Photos app
In the Finder open up the Pictures folder
Drag and drop the library to the external drive (if it's huge this could take a while)
Check and make sure it's safely on the external drive*
Delete the library from the Pictures folder
When you open Photos/iPhoto it will be empty.
How do I get to my Pictures folder?
In the Finder click Go > Home
In your Pictures folder the library that you need to backup and then delete looks like this:
*To be really safe you should also copy this library to another external drive, in case this one fails, it happens! You should also rename it and add the year at the end so it doesn't get mixed up with any others...