Charles de Gaulle International Airport is located 25km northeast of Paris. If you take a taxi from the conveniently located taxi stand, it will cost between €25 to €45, depending on where you're going and the traffic.
There is also an Air France shuttle which leaves every fifteen minutes between 05:00 and 23:00 during the week, and between 07:00 and 21:00 on weekends. Typical fare is around €15.
The RER B commuter train stops at Terminals 2 and 3. Tickets cost around €10. If you have a lot of bags, it may be inconvenient since in all likelihood you will need to transfer lines at least once.
Paris' other major airport is located in Orly, 13km south of the city. It is at the opposite end of the RER B line, and also has shuttles and taxis that can take you into the city.