Hi Paola, welcome to the world of NinjaTrader!
The first thing is to distinguish between the trading platform and the broker where you keep your funds.
If you want to trade futures, you can check out NinjaTrader Brokerage. If you prefer to use an external broker, NinjaTrader Desktop also supports connections with compatible providers, such as Interactive Brokers, FOREX.com, and FXCM, depending on the market and your account terms.
Please note that some connections with external brokers require an additional subscription. Also, funds are deposited into the broker's account, not directly into the NinjaTrader software.
Before choosing, I recommend checking the fees, available market data, and account opening requirements.
From which country do you want to trade and which market interests you: futures, forex or stocks?
With those two pieces of information we can guide you better.
All the best!