Happy New year, my name is Dan.
You need to move files off the internal drive as soon as you can.
It will not be long before it will run out of room and lock up.
OS X requires about 1 GB to run correctly.
I do suggest freeing a few GB up and getting a larger external drive to keep the iPhoto library on.
Why would you keep that many emails? Not my business but it seems unusual. I do suggest moving those also.
I do not know how long you have been getting that message but act as soon as possible, I have seen disaster by ignoring it or putting it off.
After moving the iPhoto library then the first time you open iPhoto hold the 'Option' key when you click the dock icon. This will open a window that gives you choices to choose the library.
The email will still be available, save them to a folder on the external that you create, call it saved email, or whatever you like.
Save a few and check to make sure and then delete as you save, this will free up space as you go. Empty the trash often, in fact I suggest doing that now.
Do that and let me know.