The best way to start is to get Mark Wilson's Complete Course in Magic. You could do an hour show with the material in that book. Just reading the book and thinking about how and why the effects explained work will do more to advance you magic than watching 100 hours of videos on YouTube. It costs less than $15 USD. I'm not sure what shipping would be or if you can find it in Greece or from a store in elsewhere in Europe.
To answer your question about what else to buy, my advice is to think about what you would perform most. I don't buy a new effect until I decide how I'm going to present it. If you don't have any other magic props, I'd get a thumb tip, a 12 inch silk (to vanish in the Thumb Tip), a set of 2 inch sponge balls, a couple of decks of Bicycle or similar quality cards, some standard Bicycle gaff cards like double facers and double backers (R/R, B/B and R/B) and an invisible deck or Brainwave deck.