I see three main misconceptions from new and prospective trainers:
Misconception 1 – Being a trainer means you get to work out a lot.
If you're getting into training because you believe this: STOP. Once you become a trainer, the majority of your time is spent standing, watching, correcting, and talking to your clients. You often have to struggle to fit your workouts in at 5AM before your morning crowd, or at 8PM after your last client leaves. Want to just work out a lot? Do that. Don't become a trainer.
Misconception 2 – Being a trainer automatically means big money.
False. Sure, you can charge a decent hourly wage, but money can be inconsistent. (Warning: Study up on taxes before you get into this profession.)
The most important thing you can learn as a trainer is to be a good sales person. Personal trainers are a plenty. You most likely won't have clients naturally flowing to you and begging to pay you $60 per hour consistently. Learn to market yourself, learn to stay organized, learn how to follow up with clients, and learn how to ask for money. Teaching movement happens after all that is established.
Misconception 3 – Being strong/fast/fit means that you'll be a good trainer.
This is important: The best athletes often make the worst trainers. Being inherently good at something, or "naturally" gifted athletically, often means that you're absolutely terrible at breaking down the movement to the raw basics for the less-than-gifted crowd.
Additional Tip: Before you try to become a trainer, try to teach your grandma to overhead squat with a PVC pipe. If you find yourself getting frustrated and feeling helpless after 20 minutes of failed attempts, you should probably reconsider your methods or your choice in profession.