We recommend taking the Metro to get around in Paris. The metro is cheap, fast and very convenient. The metro routes are color coded, and with a proper map they’re not too hard to learn.
You can buy single tickets, which you can use to transfer between two different metro cars, but if you’re looking to travel more than once (which you should when you’re visiting Paris as a tourist), it’s probably more convenient to buy tickets in bulk, which is cheaper. Day tickets are also available, and are even cheaper than single tickets.