That's a tough one. "Study the breed standard" is always thrown around, but the breed standard is pretty vague about some things. There is an illustrated standard available through the ABC (and the pictures are on the website), but it's of limited usefulness since it only shows *correct* conformation. There is a new judges' booklet coming out that sounds more promising. Barring that, I recommend getting the Doberman Pinscher illustrated standard along with the Boxer illustrated. Except for a few differences, the Doberman standard is nearly identical to the Boxer - and their illustrated has examples of correct and incorrection conformation. The Boxer Blueprint by Daniel Buchwald has illustrations of common conformational faults in Boxers, but it does not have quite as many examples and is out of print.
Also, go to as many shows as you can (Boxer Specialty shows are very helpful, as there are a greater number of Boxers in one place than you usually see at a non-Specialty show). Watch the judging, and rank the dogs in your mind (or in the catalog, in pencil, if you like). See how close you get to what the judge chooses (you can mark this in the catalog, too). Some judges will give you "clues" as to what they're evaluating on a dog - measuring the angles of the shoulders with their fingers, running their hands over certain areas repeatedly, lining the dogs up facing them (looking at fronts), moving the dogs several times, etc. "Overhear" what conversations you can from ringside - but remember that the information you receive may be based more on bitterness than fact! Take everything with a grain of salt, but you'll probably get a few pearls of wisdom you can hold on to. While not everyone will agree with every judges' placements, once you get to the point where you're fairly consistently placing the dogs the way the judge does, you're doing pretty well. If you can see and agree with *why* the judge made the placements the way the did, you're doing even better!
Next, talk to the breeders and exhibitors of the dogs you like (*after* judging is completed!), find out their pedigrees, think about what you do and don't like about them. Often you'll find that the dogs you like come from the same lines, and that's where you then start looking for a puppy.
Now, granted, you may not get to this point before you get your puppy. That's fine, as this is a lifelong learning process. But the better you are at evaluating correctly what the judges feel meet the standard, the better your chances of picking a quality puppy.
How do you select that show puppy?
First you need to know what it is you want from your puppy. Certainly if you're looking for a show puppy you want one that has the potential to become a finished Champion. But do you want more than that? A Specials Boxer? Top Twenty? Do you want to breed? Dog or bitch? These are things you should think about before you even pick a puppy. A Specials-quality dog certainly won't suffer from being a couch potato, but a couch potato dog will certainly be miserable if you try to Special it. Of course, there are no guarantees, but having a clear idea of what you want will help you in your quest.
You should also decide what you want from your breeder. A mentor is invaluable, and typically the breeder you purchase your puppy from will fill that role. That is not always the case, however, especially if you purchase your puppy from a breeder who is not in your area. It is usually more helpful to have a mentor nearby, and many breeders will help out newbies even if they have no connection to them. And, of course, there's no reason you can't have two! Decide on what health status/testing you will require for the parents/relatives/ancestors of your pups (hips? heart? thyroid? longevity?), what type of pedigree (linebred, outbred? number of champions in the first three generations? SOM/DOM/LOM status?) Think about what you're willing to agree to in a contract (co-ownership? breeding with a puppy back? championship requirements?).
Once you decide on the breeder and litter you're interested in, you can pick your puppy! There is a video called The Puppy Puzzle by Pat Hastings - it goes over, in detail, how to evaluate puppies at 8 weeks of age to determine their conformation at maturity. Also, your breeder will know how her lines mature, and should be able to advise you in that regard. It's hard, of course, to pick that Best in Show winner at 8 weeks of age, and there are no guarantees. In Boxers, one thing that narrows the field a bit is markings - plain Boxers, unfortunately at this time, are generally harder to finish than flashy ones, and until that changes it should be a consideration. You should be aware that white on puppies typically shrinks as they get older.
Some book recommendations (in addition to the ones you have):
The Winning Edge: Show Ring Secrets by George Alston
Born to Win: Breed to Succeed by Patricia Craige Trotter
Tips and Tricks: From Best Intentions to Best In Show by Pat Hastings
The World of the Boxer by Rick Tomita
The New Boxer by Billie McFadden
Also the AKC Breed Video is good to have.
Books are available at www.sitstay.com, www.dogwise.com, www.amazon.com Pat Hasting's book and video are also available at www.dogfolk.com (her website). The AKC video is, of course, available through the AKC (www.akc.org) but it may also be available at sitstay or dogwise.
Puppies begin showing at 6 months, although some matches will accept them at 4 or sometimes even 2 months. Typically puppies don't do a lot of winning - really the point is to get them used to the ring and to teach them that showing = fun. There are some dogs that finish their championships before they are a year old (in North America, anyway - in many other countries a dog cannot become a champion until they have won at least once at over 12 months of age).
Showing can be expensive, but it really depends on what you decide to do. Entries are usually about $20 per show (that's $20 each day of a show weekend, whether it's 2, 3, or 4 days). If you show the dog yourself, you won't have to pay handling fees (but it may take longer to finish, until you get to the level of proficiency needed). If you stay within a couple of hours' drive time you won't have to pay hotel fees, but you might not get to show as often as you'd like.