I work part time as a trainer (At fitness first - there's a few in London). We work free lance so just pay a fixed rent and basically do what we want.
To cut a very long story short I've been doing it part time as a means of testing the water before deciding whether or not this is what I want to do (it is).
So, there's a lot of things I believe I've done right, a lot of things I believe i've done wrong, a lot of things i've done that I shouldn't and A LOT that I haven't done but should have.
Where do you want me to start? lol
Anyway, in the time i've been doing it (2 years), i've seen several trainers come and go, most leaving worse off than when they arrived, some leaving in serious debt. Don't let that dishearten you though as some trainers in my gym do amazingly well. I've also managed to stay afloat and most of the time profit a little after costs despite not actively trying to get leads for over a year (I still need to pay full time costs despite only working part time hours).
I've got a few random things but if you want to ask anything specific then feel free, this should get the ball rolling.
-Certifications are crap. Sorry, but they are. They are, however, very necessary. They're mostly all about just making money, paying little attention to quality i.e. what they actually teach you. They'll baffle you by forcing you to learn how motor neurons work before learning how to teach basic practical things like a DB Row (in fact, depending on certification, you may never learn this). I'm certified via one of the alleged top 3. I'm going out on a wim and saying it's crap. The one i wanted to do (NSCA) doesn't even have a practical component, although isn't even recognised in most gyms in the UK anyway. Without ranting too much, the point is - a certification is a foot in the door. Learn it to pass it but realise that you need to keep learning, which you've stated anyway so i'm probably preaching to the choir.
-You need a process(s) for getting leads. It seems obvious but most trainers who start, think clients will just be handed to them by the gym. This doesn't happen. If your gym does supply leads, they are also generally low quality, but worth it none the less if that's all you have.
-Related to the above, it's very easy when working in a gym to hang around reception or in one of the offices. Ironically enough the fitness industry has a very lazy work ethic. Unless you are eating, or doing paper work of some kind, stay out the offices and remain on the gym floor. I've actually picked up clients whilst sitting in our Juice Bar writing programs - they've came up and asked questions, it's led to a free session, which has led to paid sessions. It wasn't intentional at first, I just knew it would be difficult to concentrate in the office. The point is you need to be visible and approachable as much as possible. One of the best tips i've heard is to always be in the gym at the times you wish to be booked and getting paid, regardless of whether you have sessions or not. For example, if you want to eventually have paid sessions from 9am till 3pm on a Saturday, then always be in the gym at that time, even if you have no sessions.
I've picked up clients during the most unsuspecting times. If I'm in and have no sessions, I float around, basically. I use this time to experiment with things, or even train. People just ask stuff. I always have time for them and always help out. I say "hi" to people i've never spoke to, and ask simple questions like, "how's things?", or, "how's the training going?". Sometimes you get, "fine, thanks". Other times you get a life story. Occasionally you get the chance for a client. At the very least you're building relationships, helping yourself become known as that trainer who is always keen to help, nice and friendly, not an a$$hole, etc.
That's mostly about just getting leads and the business end. I would recommend anything by Alwyn Cosgrove in that regard and resources like strengthcoach.com for more. However feel free to ask anything.
Learning is crucial and should be prioritised. Remember you are your own lab. I think you should play to your strength but also develop your weaknesses. You said free weights is a weakness so, if I were you, I'd start doing a free weight based training program, set some goals etc as if you were a new client and go through the process. It's one thing to read about stuff but another to actually try it. Quite often when you put something into practice, you get a whole different perspective. Sometimes things that look good on paper are crap in practice and sometimes things that look crap on paper work great in practice.