Can you actually put money in the stock market UK?

Before I ask the question I would like to explain that I'm 15 so don't insult my intelligence if this question seems stupid. I was playing GTA V and I notcied theres a stock market, I was talking to a friend and he said that anyone can put money in the stock market or buy shares in real life and you can make loads from it, is this true? If so, how old do you need to be and could you put say £5 in and get about £30 if you're lucky?