question

You begin here: https://developer.amazon.com/appsandservices/solutions/alexa/alexa-skills-kit/getting-started-guide

As Stefan linked, the Amazon docs are pretty good. I have a book on the market, it's not aimed at beginners. Also check the existing skill list before staring a project (if you are thinking of publishing). There are, already, quite a few simple demo games written. Be aware that it can't be your only guide. I have a blackjack game in certification hell because they consider it "unsuitable for all ages" even though there is no gambling involved and even though they approved of someone else's blackjack game less than a week before I submitted mine. I don't know what their criteria are for "unsuitable for all ages". They won't tell me or engage with me to explain why blackjack-without-betting is any different than go-fish. So take care in your choice of games.