You need to get out with a carpet cleaner and see the job in action,
It's easy to be an adequate carpet cleaner I'm sure your quite capable of doing a job that people will be happy to pay for. All the extra knowledge you will pick over the next few months will make you an ok carpet cleaner, then in a few years and you will be a good carpet cleaner.
Buy a tub of pure clean, rinse with a ph neutral detergent, this will keep you safe across a broad spectrum of carpets and fabrics,
You need to practice on the difficult carpets and stains but you will come across these in time, but like robin said if you are not sure just walk away.
The hardest thing is knowing when to stop cleaning and recognise that's the cleanest the carpet is going to get, you will doubt yourself and think you can make it cleaner but in reality the carpets knackered.