At the beginning you will not make a lot because you will likely have to work for a gym and they will rape you. Normally they give the trainer about 25-40% of what the gym charges, which is garbage, but that's how it works. Therefore they charge $60-75 a session and you might see $20-30. The advantage of working in a gym is that clients will come faster than if you are recruiting them on your own. It's part time income. It can take over a year for you to build enough clientele to make decent money. Going on your own means more up front but you keep what you make - it's just more work advertising, networking, getting the name out and building a referral base (your best friend as a trainer).
My own career started working for a crappy chain gym for $20 an hour and now I charge anywhere from $60-90 and keep it all. I can work 20 hours a week (4 hours a day) and make $60-75K a year. And my tax rate being self employed and writing everything off is WAY lower than most. However, it has taken years for me to get to this position, and I have spent most of that time in courses and programs so that I can learn things to make me an expert in many areas. I'm going back to university in September part time just to get some good physiology courses under my belt.
