I was already in decent shape (looks wise). Hit the weights, did some basic cardio.
Jumped in the ring, and realized I was nowhere near the level of conditioning and cardio needed to train. But I didn't give up, I just kept on training.
Don't worry, everyone starts off slow. I'm not gonna lie though. You gotta have above average cardio to even train for boxing. However, fitness clubs are a good place to start. Heck, even many hardcore boxing gyms still have cardio classes for ordinary regular people who just want to get in shape and have no desire to compete. How do you think they pay the bills?
So, my point is - I can respect the idea of getting in decent shape first, before any serious training. But many boxing gyms can help you get on board on both levels. Good luck, and keep positive. Attitude is everything in this sport.