You can buy speech recognition applications such as Dragon NaturallySpeaking, but Windows comes with speech recognition built in. You will need a headset which includes a microphone. These can be as cheap as £7.00, although it is worth spending more to get better quality.
In Windows 7 and 10 go to the Control Panel and then Speech Recognition.
Make sure you read over your work since the speech recognition isn't always accurate. You can also set up the 'Text to Speech' and let that read your work back to you.