I've no idea why someone would say trading demo is a waste of time. It's great when you first start for learning your chosen trading platform or testing systems you've either created or picked up from somewhere. It's good practise without risking real money but it's not the same as trading with a real account, when your hard earned dollars are on the line you will find your decision making process can be very different to trading a demo account.
I found it very beneficial to trade with a very small live account trading micro lots to start with ie $200 or there abouts. I doubt it will change too many lifestyles if you lose it and believe me, in the first few years you are going to make a heap of mistakes regardless of how good you think you or your system is, it's just the nature of the beast. Don't become a Forex Factory story risking your life savings in the first few years.
After trading full time for the last 3.5 years I think the best advice I could offer is to find a system/method/time frame that you are happy with, suits your personality and then stick to it, don't do what many do and flip flop around from system to system, it's a recipe for disaster.
Just be aware that regardless of what forum people will tell you this endeavour takes a long time and a hell of a lot of work, if anyone tells you differently they're either short term gamblers or liars. Consistency in trading is a tough gig!
Am I making a fortune after 3.5 years full time, no. Am I consistent after 3.5 years full time, only in the last few months and it's it's been one hell of a battle to get there but you will find that success, more often than not, will come to people that refuse to give in.