Photoshop’s native file format is a PSD. This means that any Photoshop document can be read by any version of Photoshop. PSD files preserve the layer format so the file can continue to be edited.
Go to File >Save As.
In the dialogue box, navigate to where you keep your files, either on your computer's hard drive or on an external hard drive. I recommend you keep your PSD files in a Work in Progress (WIP) folder.
In the Format box, make sure Photoshop has been chosen. Rename the file so it's name means something to you, such as "2015JaneBirthday." Click Save.