What is the minimum investment amount, and which broker is the best option for this?

Theoretically, you can start with any amount, even from $10, however, the commissions that you need to pay for transactions and the time spent on trading are not compensated for by such a volume, you should start trading from $200 and above, and I can advise DEGIRO as a broker, so as not to explain to you why this broker is reliable, see more information