If you're afraid of not getting off to a good start in class that's understandable. You could start doing pushups, skipping rope, and running, etc to get a little in shape first, (you can also practice balance and footwork drills from youtube, things like that will only help) that will help a LOT if you are very weak or have little endurance. It will make you able to learn and do things much quicker in class by cuttng down on the time you would have wasted at the beginning. Physical limitations CAN likely keep you from being able to execute basic movements that are being taught. If you are very weak, it might take 6 months but you will feel great just in the very first week or two and will continue to enjoy seeing your own progress. This has to be experienced, it is a distinct sensation so amazing that it cannot be described.
If you are average or above average athleticism, you should be fine to start right now (or take just a few weeks to get in better shape). But it's your call.
[edit] if you are very weak, I would start with wrestling. It's probably the most important for you in this scenario and can help with your footwork, balance, equilibrium, toughness and distance-judement/timing awareness later when you take boxing. Extremely useful.
If you are already athletic and strong then boxing might be the thing for you to jump right into.
