The metro is really good and easy to use. You just have to buy a Bip card (equivalent to an oyster) when you arrive and then keep it topped up using the machines that are at every station. The cost of the journey depends on the time of day, e.g it’s more during rush hour. The Metro can get very crowded so just be willing push on and be squished in with everyone. Top up cards - tube is really straight forward. 

In terms of buses for long trips, use coach company, Pullman. They’re pretty clean and generally go on time. https://www.pullmanbus.com/

When you arrive at the airport, make sure you get an official taxi and not just a random one waiting outside. Just go up to the taxi desk and tell them where you’re going and they’ll sort it for you. It seems overly-cautious but even the locals do the same. Uber is technically banned but everyone still uses it. Just don’t try and get one to the airport because they refuse to go there as they always get stopped.