I have a really good game storyline idea, what do you suggest?

The idea is based on alternate history. So it would be like historical fiction. (Think Assassin's Creed)

What next, I definitely want to make it myself, and all I know is Java.

So should I make it as an app or for desktop like Minecraft, it would probably be free or very cheap.

I know this isn't much information, I suppose I could say that it would be WWII based alternate history…

Also, what genre should I make it, I was thinking strategy, but I don't have any ideas for game mechanics in that area, but no other areas sound like they could fit with the theme.

Any suggestions are welcome.

Mystery with a bit of horror is interesting

Once you have a storyline you will need to know the following in order to make it yourself: Graphic Design and a programming language.

For Graphic Design it depends on what you want your graphics to look like. If you want a plat former game (like pacman) you will need animation software. If you want to make a 3D game (like Grand Theft Auto or Minecraft) you will need 3D modeling software as well as animation software.

For a programming language it depends what console you want to run it on. Xbox and Playstation games are usually made in C++ however, Xbox games like indie games are usually made from C#. PC games usually use Java.

Keep in mind making a video game is not a quick and easy thing they usually take years of development for example the recently released Grand Theft Auto: 5 the game looks great but Rockstar Games was developing it for 5 years.

Game mechanic:

World runs in real time, and the war constantly changes. Online or offline.

You need a better plot than that.