Don't. Dump everything in an Exchange Traded Fund (ETF) that tracks the S&P 500. The one with the lowest fees is the Vanguard SPY asset. Unless you dedicate a lifetime to understanding the market, you'll never turn a profit greater than what this ETF will give you. Beating the market is almost impossible without insider trading.
Edit: missed that you're in Italy. Find a similar asset in your country. There are similar ones on all major exchanges. You just want to make sure it's tracking the most successful companies. That's the gist of what the S&P 500 does. When a company starts performing worse than one outside this range, that new company takes its place. So the list always tracks the top 500. These are global brands, so there is almost certainly a similar asset in your country that tracks most—if not all—of these companies.
Damn, taking it way back with this one.