It honestly really doesn't matter. Since you're a beginner, I would recommend XY because it's definitely the easiest and does a lot of hand holding throughout the game, but at the same time, I don't want the worst pokemon game to leave a bad taste in your mouth so you don't want to try the rest. If you want easy go for XY. If you want to enjoy it, go for almost any other one. Omega Ruby and Alpha Sapphire are the best pokemon games ever made, so that's another really good place to start. Or, else you could just start from the beginning, and play RBY or FR/LG. It really doesn't matter. Just chose what looks funnest to you.
Edit: Another thing is that in FireRed/LeafGreen, if you click L or R, there's a menu basically giving you any tutorial information you need and you can click it any time you need to. It shows which types are strong against other and other information like that, so it would be really helpful for a beginner like you.
