As has been suggested, the time-honored procedure for inexperienced instructors in most US ski schools is the "hiring clinic" in the fall. Do not mistake this for Hollywood's hiring clinic in Aspen Extreme--it's not quite like that! It can be anything from a one day casual thing to a multi-day, intensive process that includes some job training. Either way, here's what you need to get hired at most ski schools in the US:
1) A great attitude! You have to be able to demonstrate to the hiring clinicians (usually supervisors or senior staff and trainers of the ski school) that you love working with people, that you are friendly, and that you have contagious enthusiasm for helping people enjoy the sport of skiing. You need to both love skiing, and love helping others share your joy. You should not have to work at this, because it should be the obvious truth, on both accounts. Believe me, you don't want the job if it isn't!
2) Interpersonal and communication skills--both verbal and non--to support your desire to help people. Desire is NOT everything--you have to bring some ability to the table as well. The clinicians will test you--they'll observe to see how you communicate with them, with other people on the slope, and with lift operators and other employees. They may put you in front of the group to see how you perform (although they will not expect you to know much about the technical side of ski teaching--and don't try to fool them if you don't). Are you multi-lingual? Do you have any experience or education in teaching? Do you have other life or job experience or education that would help you teach skiing more effectively? Bring these up!
3) Professionalism. Do you present yourself in a way that would reflect well on the ski school and the resort? Are you presentable? You don't have to dress in the snappiest and latest outfits, and have brand new equipment, but your clothing should be appropriate for skiing and reasonably clean and untorn. You should look like a pro--shaved or recently trimmed, and so on. If you show up looking like a dirt bag, they probably won't even give you a chance for a second impression. Learn and obey the rules of safety and etiquette for the slopes, ski safely, and above all else, do not scare the clinicians, or beat up any guests!
4) Last, and possibly least (which may come as a surprise to some), you need only a modicum of skiing ability. While teaching high level students requires strong skiing skills, teaching beginners does not. Great learners usually make better teachers than great skiers who have stopped learning themselves. More important than your current ability, for most ski schools, is your desire and ability to learn. Are you coachable? Of all the qualities that make great ski instructors, skiing skills are the easiest for the ski school trainers to teach you. Show them that you love to learn--all great teachers do!
That's about it. Talk to the ski school directors at whatever resorts you're interested in working at, and find out when their hiring clinics are. The more flexible you are, both in what you can do--kids, adults, seniors, snowboard, terrain park--and when you can be available, the better (especially for part-time instructors).
Once you are hired, plan to do some serious learning. You will probably revisit your skiing skills from the ground up. They will coach you in current teaching theory and methodology. They'll teach you any specifics that are important to the ski school's individual philosophy and particular needs. There will be inhouse learning opportunities, some required, some optional. Some ski schools, especially the large ones, have extensive training programs that will take your skiing and teaching skills to very high levels. Others have only minimal programs.
Plan to join PSIA (in the US), or the national instructor education/certification body of whatever country you are working in. In the US, it is not too difficult to become certified Level 1 in your first season, if you attend a few clinics and put some effort into it. Full Certification takes several years, usually, so get started on it as soon as you can.
It's often said that teaching skiing is a great life, but a lousy way to make a living. Don't get into it for the wrong reasons! No one's ever gotten rich teaching skiing, and you'll have to watch from the beginner hill as others make first tracks on more than a few great powder days. But if you're right for it, nothing else will ever be so rewarding!
Most importantly, keep us all posted on your progress! There's a lot of good help here at EpicSki--lots of instructors of every level of experience, and lots of skiers to bounce your ideas off. And I, for one, am eager to hear how it goes for you. Best of luck!