How to know if I can use my new game title legally? And its not copyrighted.

The answer is different for every country. Assuming that you're talking about the US...

First, you should understand the difference between copyrights and trademarks. Your game is copyrighted but the title would be a trademark. Similarly, Pepsi is a trademark. To do a trademark search, go to the US Patent Office.

Also, you need to search on whatever store you plan to use for your game, as there may be apps that use the same name that haven't registered a trademark.