As for college, certification, etc...
The quality of training through Petsmart varies so much. I worked there for a time, and met a few different area trainers--area trainers train the trainers in each store in their area, just to clarify... 2 were worthless. 1 was incredible. She was a training geek like most of us here, had millions of books, etc, then
went to Petsmart's training, where she happened to learn under a good area trainer. That, combined with lots of work with various types of dogs, got her to where she is now. The other two, like many Petsmart trainers, were just people looking for jobs. They found a job as a trainer, went through the training program, were good employees, and got promoted. Of all the Petsmart trainers I've known, the majority have not been impressive. But, that was all in Texas. I was thoroughly disappointed with the Petsmart training program, simply because if you aren't someone who is passionate about dog training/behavior in the first place, you won't get much out of it. Of course, if you happen to learn under a good area trainer, AND you're passionate about training and do look for information outside of just the course, then you'll be great.
My point is don't count on training programs of chain stores to make you much of a trainer. Both Petsmart and PetCo have some that are outstanding and some that are just terrible.
Bergin University is the college I've really been looking into; it is not expensive and I've heard nothing but great things about their programs. The downside: it's in California. But learning about their programs and talking to some of the alumni will really rope you in. I'm in love with this school and still may be going there after finishing my schooling at my current university. Even though California is the last place I would like to live.
I haven't heard much about ABC but I do get their monthly emails because I emailed them with some questions. I haven't heard anything from alumni or anything as far as the quality of their programs or their repuatation. But, they have answered any question I had for them and have been helpful.
Volunteering at shelters is one of the best ways to get experience with behavior and training if you don't have a mentor. You can gain experience, put to use what you have only read in books, and help dogs find homes all at the same time. Plus you don't have to worry as much about being judged. Once you've kind of established yourself to shelter employees and other volunteers, you might make out a business card or something to give them so that they can be handed out with adopted dogs. And once you've established yourself to shelter employees and volunteers, they might have dogs of their own they need help with or have friends, family, etc that might need help. See how many connections and opportunities you could get through volunteering at shelters?
You don't necessarily have to go in saying you want to train all their dogs, just volunteer as a dog walker or whatever other duties for a while, and once they know you a little better, talk to one of the higher-ups about doing some basic obedience work with some of the dogs. Then just let it build from there. If you just come in saying, "I want to volunteer here and train all the dogs," they're likely to be skeptical--you're young, and just because you say you can train dogs doesn't mean you can. And they can't have faith in you and your methods without seeing them. Even if they believe you are capable of training dogs, they may not necessarily believe that you use the methods you say you do.
I volunteered at the shelter I adopted Mud from, and worked with tons of dogs there. After adopting Mud, I would bring her to various shelter events to show her off and do little trick demos. This helped in a few ways: promoted adoption from that shelter, got my name out, and helped the shelter owners and employees trust my abilities more.
It might be a good idea to pick(through volunteering as a walker or whatever and getting to know the dogs) a dog that might be a good candidate for this type of work. Relatively confident, trainable, intelligent, etc...so you can maybe teach this dog a few tricks and show him/her off at events in the same manner. High-five, shake, kiss, beg; these are all easy tricks to teach that people just eat up. When this dog gets adopted(which he/she surely will with your training behind him/her), find another.
Most of my clients start out needing help with basic obedience issues. So I'll start with the basics: sit, down, stay, come, wait, leave-it, leash-training(very important...if the owner can't walk them comfortably, 99% of owners just won't...then they have more issues--they might become return clients, or they'll trashtalk you and your training, or worse, give up the dog), no jumping, no biting, etc. Make sure the owner understands very clearly how to implement your methods, so that when you're not there they still know how to handle problems. See what else specifically they want the dog to know and add that in too.
There are 3 or 4 posts stickied in the puppy forums that I typically hand out to new puppy clients; feel free to print them and hand them out if these are methods you agree with. For me, it makes it easy because if I'm not there they can refer to the article and then ask me questions if the article didn't answer it. If they can't get it right with me telling them how, showing them how, and having written references...then they are just lazy and don't want to put forth the effort to correct the issues they're having.
Hope this helps.
Congrats on the new student and good luck!