TomasRoj:
Hey, when you started with arduino, what was best example project that helped you most?
I don't see that as a realistic question. It all depends on the project you are trying to implement.
For example there would be little point in learning all about I2C if your project does not need it. Indeed I never used it until about 2 weeks ago.
Experience counts for a lot. Every time you run into an error and figure out the problem your stock of knowledge grows.
Back in the day there were monthly magazines with lots of program examples for things like the Commodore 64, the Sinclair Spektrum and the original MSDOS IBM PC. I learned a great deal from reading the magazines long before I could afford a Spektrum. IMHO the same learning can be achieved now by reading lots of Forum threads. You probably won't remember all you read but when you see a problem your brain will say "didn't I read about something similar to this ? - now, where was it?"
...R