Well after completing an accredited course you should definatley have the knowledge and practical know how for the trade. I'll be honest with you not everyones cut out for tiling for various reasons such as impatience, lack of concentration, personal skill levels, and quite frankly just not having the eye for it.
Only you and your tutor will really know if your good enough to now earn a living from what you have been taught as you will have had by no doubt assesments of your work during training. Best thing is to be Be honest with yourself and think about the work you have completed at the latter end of your course and ask yourself honestly if you would pay for the standard of work that you are currently capable of. If the answer is no don't waste money on advertising etc and definatley don't do any jobs with paying customers just yet as you will only end up with a bad reputation, if possible just put a little more practice in with family and freinds. But if the answer is yes then go for it and if its just a case of building up your confidence don't take on anything to big just stay with the smaller jobs for now. You would also benefit from contacting networking groups in your area such as the BNI you can find details through there website. This will help you with the interaction side while also building relationships with local businesses and you will find the confidence will come naturally.
All the best
Kenny