There is real life and then there is movies. You were influenced by a movie which I am sure happened to all of us one way or another.
The thing about training whether boxing, MMA, whatever most likely you will be training on your own, thinking you are making progress, believing you have good technique, then you walk into a gym and deal with a trainer and/or go into a sparring session and then you get a reality check. The fact is to properly learn a skill (boxing, MMA, etc.) the only real way to get good at it is getting properly trained. Even if the desire will only be a hobby what I am saying holds true.
If you want to get an upper-hand till you can get properly trained is get yourself in the best shape you can. Eat right, work a lot on cardio, some strength, and stay consistent. Also, try to do exercises that you might do in a boxing gym for instance: example jump rope. Even if you were to do Burpees or Manmakers you would be in a better position than if you were to sit on the couch waiting till the day you can get to the gym.
To summarize, get fit, stay fit, and when you walk in, at least you are not walking in completely raw. Stay humble and take advice, and learn. Be a sponge and absorb as much as you can.