You need to apply a thickness to everything, including the hair. Right now your model is “non-watertight” but the important thing is you need to turn it into a solid. 3D printers only understand solids. I don’t know how to do this in Blender.
You need to give supports to the hair if you’re using a FDM/FFF printer (I’m assuming you are but you might not be).
You need to do boolean unions of all the objects in your model (AFTER you’ve given them a thickness), including supports. This is found under “Add Modifier,” which itself is found by first making sure one of the objects you want to do the union with is selected and then clicking the wrench icon (this is a small icon found on the right side of the screen). Keep the same object selected and perform a union again to add the third object. Keeping the same object selected, you can now generate an STL file. You can always tell if you have the right thing selected by looking at what gets highlighted in the main view. If it’s highlighting the whole model, you’re good. If it’s highlighting only a piece, then this is not what you want to generate the STL from because you’ll just get that piece.
Actually I’d removed the hair and eyebrow and fixed most defects like non-manifold. But still there’re many “overhang” faces that confused me. So I threw the original file to make sure if I started wrong.
Here is the modified head with lots of overhang faces. How can I fix that?