There are a great number of airline companies which provides flights to Paris. If you are prepared to search some for the best offers, they usually are found at reliable travel agencies.
If you are planning to book a cheap flight at some small travel agency, make sure that you really get the money back if the agency should close down the business. And don’t pay the full price before you got the ticket in your hands.
The airport of Roissy Charles de Gaulle handles most of the international flights. This airport is ranked second in Europe in terms of passenger traffic, only behind the Heathrow Airport in London. There are many taxis outside the terminals of Charles de Gaulle who willingly take you to the central of Paris. The journey lasts for about 45-50 minutes, depending on the traffic. One cheaper and perhaps more flexible alternative is to get the express train (RER) to the station of Garde du Nord or further into the city. The price is about €6 for an one way ticket.