If you want to implement your own Alexa Skills for your Amazon Echo and don’t intend to make them public, you can use a self-signed certificate for your web service where you host the skill.
I’ve been impressed on how many developers can’t make this work and have opted to use a Lambda function as a proxy, when it is very easy to create the self-signed certificate.
If you intend to publish your skill then you’d need to buy a SSL Certificate. These steps won’t help you. You also need a real and trusted SSL Certificate if you want to host audio files to be used with the Audio SSML Tag.