Text that you insert in an Emacs buffer lasts only as long as the Emacs session. To keep any text permanently, you must put it in a file.
Suppose there is a file named test.emacs in your home directory. To begin editing this file in Emacs, type
C-x C-f test.emacs <RET>
Here the file name is given as an argument to the command C-x
find-file). That command uses the minibuffer to
read the argument, and you type <RET> to terminate the argument
Emacs obeys this command by visiting the file: it creates a
buffer, copies the contents of the file into the buffer, and then
displays the buffer for editing. If you alter the text, you can
save the new text in the file by typing C-x C-s
save-buffer). This copies the altered buffer contents back
into the file test.emacs, making them permanent. Until you
save, the changed text exists only inside Emacs, and the file
test.emacs is unaltered.
To create a file, just visit it with C-x C-f as if it already existed. This creates an empty buffer, in which you can insert the text you want to put in the file. Emacs actually creates the file the first time you save this buffer with C-x C-s.
To learn more about using files in Emacs, see Files.