How to train a boxer dog for first time owner?

my dad and me have decided to get a gsd or dobe or boxer puppy,now how to train it?i am training but dads Friends bull mastiff trainer will advice me through the phone.how much to exercise it,i go to school at 6 am and come back at 2 pm,mom will be there with him all the times.how much to exercise it 30 mins in the morning and 2 hrs in the after noon??? enough for him.are boxer puppies destructive???people tell that boxer are good for my age some say they are not,some say gsd, two said dobe.which one to go for.i would like answers from these three dog breed owners.thanks