It is going to be my first year doing halter. Do you guys have any tips on how to train for this. Also, how can I work on getting my horse to always stand square?
Firstly, Welcome to the forum!
At your first year showing halter, definitely look into this as a learning experience. What sort of shows are you looking at? Small local or schooling shows? AQHA breed shows? 4-H or other organization sponsored? Knowing the circuit will go a long way to understanding what you're going to be up against and how to prepare. AQHA is a totally different animal than local playday shows, and it shows in the kind of prep that is done. Get your feet wet and get a feel for what's going on. Go as a spectator, talk to people and hear what they have to say. Horse people love talking about their horses, their disciplines, and anything else that goes into the package.
Keep in mind that Halter judges the horse: if your horse does not have stellar conformation, he's going to be less competitive, depending on the scale of the show. Be aware of his conformational flaws (every horse has 'em), and take them into account as you prepare. Any scars will count off a bit, although most judges recognize that horses do get into scrapes and don't dock as much as say, for sickle hocks. A good way to think of what scores well in halter is what you would most want to breed; the best representative of the breed, what most closely approximates the ideal Quarter Horse (although purely Halter-bred QH's do look rather different than a reining or cutting bred QH).
I personally recommend showing in Grooming and Showmanship as well as Halter, if the class is offered and it isn't outside your budget. G&S places the emphasis of judging on the person, and how well he/she has groomed and conditioned the animal, as well as their showing ability (is the pattern smooth and well-executed, is the handler confident/knowledgeable, is all tack properly maintained and adjusted and suitable to breed/type, etc.). Getting in the ring is always good experience for you and the horse, and prepping for one is basically prepping for the other.
A lot of prepping for in-hand classes (G&S or Halter) is in basic good care, management, and exercise. No amount of Showsheen is going to make up for what religious grooming and good diet and exercise do alone. If you're looking at big-time AQHA breed circuit stuff, or otherwise being uber-competitive at higher levels, then, as Ripper said, you're looking at some more specialized conditioning steps to maximize muscle, minimize fat, and really "sculpt" the horse. If you're showing at simple local shows, just riding your horse often and in a way that promotes proper musclular development is plenty (i.e., no running around inverted, exercise both sides of his body, etc.). Basic good riding shows in a properly-muscled horse for his breed and job.
You want the horse to be well-versed in leading - you don't want to be dragging him around or him dragging you. He should be quite willing and responsive to your cues in-hand at walk and trot. Extreme precision in leading isn't as vital in Halter as in Showmanship, but it's easier to show your horse to the best advantage if you have excellent ground control. I don't have time right now to lay out the nuances of squaring up or watch the video, but I'm guessing it's a pretty good overview. QH's stand square, no parking out, no hind-foot offset. If at all possible, get your hands on a copy of the organization's rulebook for the show -- that will tell you exactly what the judge should be looking for and how to prepare.
Make every occasion you lead your horse an opportunity to polish your skills. When you turn him out, make him stand square before you unsnap the lead (teaches some patience as a bonus). Have him stand square before you tie him up. Make all your turns accurate turns on the haunches, not just lazy curves to get the job done. Find little opportunities and make the most of them; let the showring way become habit. That applies to you, too -- carry the lead like the judge is watching, keep smiling, etc. If it's already habit, its harder to forget to do in the ring. Any patterns that you'll need to do in the class should be posted well beforehand. Halter doesn't vary much (walk through the gate, pick up a trot as you pass the judge, make a big loop around the ring, and take your place in the lineup, stand square, and wait for the judge to inspect you individually there), but G&S can be pretty involved depending on the level and the judge. If you can work with a friend or coach experienced in the discipline to really nail the technique and poise, that'd be excellent and really fasttrack you and your horse to success.
Most importantly, have fun!! That's the whole point, after all!