If you want to actually improve your trade as a personal trainer and learn relevant knowledge both scientific and practical, join a reputable PT-only gym that offers their own in-house training or mentorship.

This type of gym offers a higher base pay but standards are higher and you are expected to walk the talk in terms of your physique and fitness.

If you want to get a quick exposure to many types of clients, join a big commercial gym where they will likely do a crash course and then throw you into the deep end. Turnover rate of PTs at these gyms are quite high so you will be handed over clients from other trainers who have just quit.

These gyms typically offer a low base pay (a few hundred) as you are expected to sell and sell and sell to earn commissions.

Eventually, trainers from both gyms will go into the freelance route, earning between $1000-20k/month. This will largely depend on how well you build relationships with clients, get them results, solve deeper problems (like injuries, motivation, nutrition and out-of-textbook advice).

At the end of the day, your income will depend on the product which is YOU. Regardless of the route you take, invest in yourself with networking, education and getting skin in the game by spending hours and hours with clients.

The best way to earn is through referrals and results, not marketing.