Hello Traders,
after a few years pursuing trading more or less as a hobby, I want to take a step forward and improve my overall ability.
Does anyone know some good books, websites ( links) etc. on how to build a proper trading strategy and therefore a good system?
Thank you very much and have a nice weekend wherever you are!
one great book which was written to answer that specific question, and does so very well, is Trade Your Way To Financial Freedom by Van Tharp
Whatever system you use, it will have inherent issues.
Firstly, it will not be 100% effective. If 70% of your trades are winners and 30% are losers, the 30% can blow your account faster than the 70% will make you rich. So plan and demo trade and practice for the losers at least as much as you learn about the winning strategy rules and patterns etc.
Secondly, the system you go for must match you as an individual. Its hard to exactly divide them but analogies might demonstrate what I mean -
are you a sprinter or a marathon runner?
are you a chess player or a fencer?
are you an F1 driver or team engineer?
if you think you might lose money tomorrow, will you lie awake worrying about it?
The choice of your first strategy is unimportant. Its not going to make you rich anyway, but it will teach you two things - the sort of strategy you personally can live with and why a particular strategy works here but not there. These help you find your second strategy - and that one might make you rich.
Thank you 
I would suggest that you make some research on all trading methods and find one that suits you the most. Then try to perfect this method in order to achieve best possible performance. Demo accounts are very good for this purpose