In choosing a new puppy, the practices of the breeder are of paramount importance.
In the first instance, what you need to check for is that the breeder is screening their breeding stock for a number of serious hereditary diseases that are
common in the boxer breed. Those diseases are arrhythmogenic right ventricular cardiomyopathy (ARVC, aka boxer cardiomyopathy), aortic or sub-aortic stenosis, hip dysplasia and hypothyroidism.
In case it is not immediately obvious why this is so important - the reason is that these are diseases that are increasingly
common, and that are devastating. They're the sort of thing that makes it likely that your dog may drop dead without warning aged 3-5, or else be diagnosed, but need very exensive treatment for the rest of its life. Once you've spend 10-20K on treatment for a dog, you start to see how why its not really worth trying to save a few bucks on initial outlay in buying a puppy if it means you take a greater risk of having to deal with the serious stuff later
So that is the most important thing. Anyone who is breeding boxer dogs without screening them for those diseases is irresponsible, and not worth taking a risk on. Before you ever see a puppy, you should FIRST see the results of the parent dogs test results. If they're not available, they haven't been done, and you should RUN away and find a better breeder. More information about the health issues that need to be screened for here:
http://www.boxerworld.com/forums/vie...th-testing.htm and an idea of the potential consequence of not insisting on this here:
Backyard breeding - the consequences
Less critical, but also important is choosing a breeder who is breeding with proper knowledge of the breed standard - and hence going to produce dogs that look and behave as boxers are supposed to. Generally, this means buying from someone who shows their dogs. Showing, of course, being the best means out there of determining if a dog is correct enough for its breed to be used as breeding stock (that is what dog shows exist for). There aren't too many people in this world who're objective enough to assess their own dogs accurately

So if the breeding stock aren't shown, there should be a good reason why not (and preferably some other means of getting an objective external opinion on whether or not the parent dogs are actually breed-worthy).
I lose count of the number of people who don't want to buy from show breeders because they "just want a pet" - and then are posting in disappointment (or even outrage) some time later because their dog doesn't look much like a boxer and people keep asking if its a mixed breed... The way to avoid this is to buy from someone who has had their dogs assessed for breed worthiness in the first place, and NOT from some fool who's put their own two pet dogs together without regard for their suitability.
Once the above two criteria are satisfied, you should look for a bright and healthy puppy, whose breeder sells her with a health guarantee (pref. +3 years), a spay/neuter contract, and the requirement that you would return the dog to its breeder if - for any reason - you're unable to keep her at any time of her life. With that, you've a pretty good clue that you're dealing with a responsible individual who is likely to be taking all care to produce quality, healthy animals that you can have some confidence in. I would avoid the sorts of irresponsibles who don't take that sort of care. Wanting "just" a pet does not reduce the importance of any of that