Plenty of reading to get you started:
So You Want to Be A Fitness Professional
Straight Talk About the Fitness Biz (It's Part 2 of the above)
Becoming A Personal Trainer
5 Ways Trainers Need to Improve
Trainers Talking Shop (a thread I started in, holy crap, 2006 that went on for a few years with some good discussion along the way.)
Trainers Talking Shop Part 2 (More recent but short-lived sequel to the above.)
Other than that, I'd say go into being a trainer knowing that you'll start off working with a variety of clients, not all of them "ideal", but that kind of variety and challenge is going to benefit you in the long run. Even if training college-level volleyball players is your dream gig, you'll build skills that carryover by training an 80-year old retired accountant with a bad hip and a 40-year old mother of triplets.
Don't fall into treating clients like mini-versions of yourself. If you like deadlifting for sets of 10 and you like ending every workout with a bodyweight finisher, that's fine, but that doesn't mean it's appropriate for all clients. Too many new trainers think they can just put clients through the same workouts they do themselves. That's lazy and unprofessional.
You've been lifting for a few years, so you probably have some ideas about what works and what doesn't, just remember to be open-minded and keep learning. You should be able to see the pros and cons of all training methods/techniques. You, personally, might hate CrossFit or kettlebells or BOSU balls or whatever, but don't carry that bias over onto your clients.
Always keep a client's needs and goals in perspective, and remember the personal in personal training. How you communicate is as important as what you say. Some clients want their workouts to be sweaty therapy sessions where they vent about work, family, whatever.
Collect testimonials as soon as possible. Whether it's before/after pics or just a few sentences with someone saying "Gretch is the best. She got me to my first pull-up", build your portfolio just like an artist or architect does with their finished products. The "products", in this case, are clients who reached their goals.
Whether you're going to be working in a gym or plan to go on your own eventually, remember it's a business and there's more to it than just nutrition and exercise. Don't neglect "traditional business stuff" like finances/accounting to advertising.
From a business perspective, social media can be awesomely helpful or a distracting waste of your time and attention. Again, whether you're going on your own or working for a gym, I'd set up a professional page on one or two outlets as soon as possible. It'll give you another way to communicate with clients and will help connect with other trainers.
Lastly, stick around the forum and sharpen your skills by answering some questions, particularly in Beginners. It's a good way to keep yourself sharp and learn how to address new coaching issues. (Everything from dealing with new scenarios to figuring out how to get your point across.)