- Have you made a game before?
- Do you have experience with the operating systems on all the platforms you are thinking about supporting?
- Are you comfortable with getting into the guts of setting up a proper build system and understand the compilers you'll need to use for each platform?
- Do you know which language features are supported by the compilers?
- Are you using libraries which are available on all platforms?
If you've answered no to any of these questions, you almost certainly should not try to make a game that is cross platform. You'll waste so much time and energy on making something work on all the systems rather than finishing a game on just one of them.