Using the buses is a great way to get around the city, you can use metro tickets or passes on them and of course the best thing is that you are above ground and get to enjoy the views while you travel. Some of them are almost as good, if not better, than an expensive sightseeing bus.
My favourites are the 42 (start at the Gare du Nord and get off at the Champ de Mars):
and 69 (pretty much the whole route):
If you are using individual tickets make sure you insert them into the machine as you enter the bus. You can also buy them from the driver for 2€ if you don’t have any, but it’s much cheaper to buy a carnet of 10 tickets at the metro station.
You can buy a small book with all the routes in, the Paris Bus Book, otherwise check at the bus stop, or just jump on and get off when you have had enough! Remember to board at the front and exit through the rear doors. To get the bus to stop, press the nearest red button, which will signal to the driver.
For advance planning check out the RATP bus website. Click on where you want to go and it will show you the available bus routes.