On the go
I like to explore cities on foot - without the bus or train. This has the advantage that I get to see a lot more of the cities. That's why we also explored as much as possible on foot in Paris. But the metro is the main mode of transport in the French capital for a reason.
The city is so big that some routes would simply take hours on foot. I would therefore recommend that you buy a metro ticket. That way you can walk between nearby sights and take the metro to everything else.
It is cheaper if you get the navigo card. You can get these at the ticket machines in the stations. There you can also compare the prices between the navigo card and the normal tickets on the notice boards.