How much do you need to start investing and which broker is better to choose for this?

Theoretically, you can start with any amount, even with $10. But such a volume does not compensate for the commissions that will have to be paid for operations, nor the time spent on trading. It is worth starting investments, from $ 200, and you need a reliable broker, you can use DEGIRO, so as not to explain to you why it is a reliable broker, see more information about it