How can i design a video game?

I have always wanted to design an online rpg covering the harry potter story but am not sure how to go about this without a degree. Also, if u give me the link to a program please inform me of the cost with your link.How can i design a video game?
I have to. Unfortunately i have to say it is not easy at all. actually it all really depends on what you want to design. from what i've seen there is nothing simple out there. There is simple animation stuff (aniboom) but nothing specifically for that.