Everything that I'm saying presumes that you're young, and won't need your money back for 20+ years, and that you're going to invest additional money in the future.
Your first investments should never be individual stocks. That is far too risky until you have a LOT more experience in the market. (Once you absolutely can't resist, keep it to under 5% of your total investments. That lets you experiment without damaging your returns too much.) Instead you would want to invest in one or more mutual funds of some sort, which spreads out your investment across MANY companies.
With only $50, avoiding a trading commission is paramount. If you were in the US, I would recommend opening a free online brokerage account and then purchasing a no-load commission-free mutual fund. TD Ameritrade, for example, publishes a list of the funds that you can purchase without commission. The lists generally include the type of fund (index, growth, value, etc.) and its record of return.
I don't know if Europe has the same kind of discount brokerages / mutual funds the US has, but I'd be a little surprised if it didn't. You may or may not be able to invest until you first scrape together a $500 minimum, but the brokerages often have special programs/accounts for people just starting out. It should be possible to ask.
One more thing on picking a fund: most charge about a 1% annual expense ratio. (That means that a $100 investment that had a 100% gain after one year would net you $198 instead of $200, because 1% of the value of your asset ($200) is $2. The math is much more complicated, and depends on the value of your investment at every given point during the year, but that's the basic idea.) HOWEVER, there are index funds that track "the market" automatically, and they can have MUCH lower expense fees (0.05%, vs 1%) for the same quality of performance. Over 40 years, the expense ratio can have a surprisingly large impact on your net return, even 20% or more! You'll want to google separately about the right way to pick a low-expense index fund. Your online brokerage may also be able to help.
Finally, ask friends or family what mutual funds they've invested in, how they chose those funds, and what their experience has been. The point is not to have them tell you what to do, but for you to learn from the mistakes and successes of other experienced investors with whom you can follow up.