How do you talk to my apple computer and have it write down what I am saying?

Open System Preferences in your Dock. Left click and open Dictation & Speech. Now turn Dictation ON. Simply press the fn key twice when you are ready to talk. When finished click Done on the speech icon that will appear. You can change the key you use to activate the program. Speak plainly and slowly at first. It will take time for the program to learn your speech pattern. The more you use it the better it will get. Of course you must have the built in microphone turned on or be using an external Mic.