I am developing a game on Android and I was wondering if I have to copyright it before putting it on the store (so no one can steal the idea)?
While this depends on the country in which you are publishing, as not all countries will have the same copyright laws/regulations, copyright is implicit. There is nothing to register specifically to mark it as copyright unlike a patent which has to be applied for.
The difficulty comes in asserting your copyright. In the event that someone does infringe your copyright it's always good to have proof of when your implicit copyright protection took place. However the date of first publication on a store will suffice and any records of development, i.e. dates on your source repository.
Keep in mind a few things though. Copyright will essentially protect you against people taking your game and either using the assets from it or hijacking it via a straight clone. However, game mechanics and general ideas are not generally enforceable via copyright laws. They may be via patent law, but even then it's probably unlikely.
This is too your advantage. If this weren't the case, think how empty the games market would be. Only one company would be able to produce FPS games without paying a license fee to the copyright holder.
So long story short, if you want to have a market advantage, be the first to publish and market the hell out of it. Don't worry about people stealing the idea. If it's a good one, they will anyway and there isn't much you can do about that.
You need to go through steps and be careful at few steps as well while publishing app on play store. Refer this link for more details: