Nothing guarantees your success in trading. It's a difficult occupation. You can increase your chances though, but being disciplined and organized, learning as much as you can and taking as few risks as possible.
I think it is important to pick casinos with proper license from a gambling regulator to ensure that they are safe. The gambling itself is a matter of luck.
True, experimentation is often overlooked. You got a new idea about a strategy? There's absolutely nothing holding you back from testing it on demo. If it works, it works, if it doesn't, you won't lose any money.
It can also help experienced traders test their strategy and their EAs in a safe environment, so they do not lose money. People often underestimate how useful demo can be even to traders who aren't newbies anymore.
Nobody can trade on all currency pairs and be successful. Pick a few of the more popular ones and try them out, then narrow them down to one or two at a time.
The main criteria is regulation - if the broker has a proper license from a reputable regulator, then it's important to see whether their trading conditions are good.