I like to use Emacs as my text editor. You may like something else better. It's possible you'll need to install it on your computer. If so, visit http://aquamacs.org/ or www.gnu.org/software/emacs/ depending on your taste and whether you have enough privilege to install software on the computer you're using.
You will see a window pop up with some information about Emacs. Click in the window to make the information go away. Let's start a new file. Type control-x followed by control-f. Now, at the bottom of the window, you'll have the opportunity to type the full pathname of any file on your system.
Hit return. Now type some text into the window. You can use the mouse or arrows to move around in the window just like with any word processor. When you're done, save the file by typing control-x followed by control-s.
You should receive a confirmation message at the bottom of the window that says
Wrote (whatever you named the file)
Your file is saved. If you are finished with this file, you can "kill the buffer" by typing control-x followed by k (not control-k) followed by return. Or, under the "Files" menu choose "Kill current buffer".
Emacs is a wonderful text editor, but there are many others. If you have a favorite, by all means use it. There are many Emacs tutorials on the Web. Just search Google for "emacs tutorial" and take your pick.
Just so you know, if you start typing a command in emacs and you mess up (by typing the wrong control character or something like that), you can usually get out of the command by typing control-g.