I have been boxing for 8 years and still compete at a high amateur level, I also took up NHB (well the grappling aspect) to compete in the cage as well.
It depends on your fitness level.....I have been doing it for so long now that even while bulking I still do 3 hours of boxing 4 days a week....because of this I have never stepped foot on the cardio machines at the gym.
like I said I dont know your fitness level, so I will say you are a newbie in ok shape
start off with a warmup....do some jumping jacks and run on the spot for 15 minutes, do some pushups and situps 50 each is a good start.....then stretch....do trunk twists....stretch your shoulders and forearms/wrists (hold arm straight out, bend wrist down till you feel stretch then bend up and do the same)
now is a good time to do some light shadow boxing...just do 3, 3 minute rounds and stay loose.....now hit the heavybag....3 rounds...3 mins each...dont stand flat fotted in front of the bag....move like it was an opponent and throw combo's...STAY ON YOUR TOES....bouncing.....dont try to throw everything you got into it....throw quick shots with medium power, snap the jab etc. I see new gys come in all the time...jump on the bag and throw hooks like they think they are tyson.....straight punches set up the powershots...also work the body and head. Alot of new guys also cant space themselves from the bag.....you want to snap the jab....the bag is at the end of the punch...some guys are too close and they think they are just full of power when all their shots are really more of a push. also use handwraps and good bag gloves...not those skimpy leather gloves that offer no padding or support.....I use 16oz bag gloves. Depending on your weight choose the right bag (if you have the option but a good gym will have the right equiptment) If you are 190-200 pounds...try to find a 200 pound heavybag.....if not use a 150 pound bag minimum.
If you have a speedbag, practice until you are good...then speed up and use power....smaller the bag the faster it goes, keep your hands up and dont only use one arm mix it up...right/left...right/right/left/left etc. its a great excersise for the tri's and it helps with rythm....the double end bag is much better though for hand eye coordination.
do another 3 rounds of shadow boxing....to loosen up again and dont forget to throw combo's...imagine an opponent throwing back....slip/parry play defense as well as offence and move around dont stand still.....circle with the jab....pivot and use angles
If you were on your toes the whole time bouncing.....stretch your calves....trust me until you get used to it.....they will hurt the next day.
as you get better go longer and up the intensity....but only with proper form, find a good gym and do some light sparring as well.
This is what I would get a guy to do his first week and take it from there.
Any questions let me know but try to find a good gym if you can
As you get