You need a SUBE card!
SUBE is a transit card used to pay for all public transportation. You can buy the card and add funds to it at any metro station. Also most kiosks and lottery outlets sell and recharge SUBE.
You can now pay with Apple Pay and / or debit / credit card on subway and buses. However, some cards may not work so it's advisable to have a SUBE with some credit on it just in case.
Tip: Use this map to get from Point A to Point B. It’s like Google Maps but designed for the city of Buenos Aires! http://mapa.buenosaires.gob.ar/
You just pay once and you can combine lines inside. Here you will find the map of Buenos Aires subway lines:
If you prefer to get around the city by taxi, it's advisable to take an app-car rather than flagging a taxi on the street. Here you can find the most well-known options: