There are several methods, and the answer depends a little on your final destination.
The RER is a suburban rail system, and there are two terminals at CDG. You’ll see sigsns that say something like “Paris par train” that will direct you to one of the terminals, and the clerks who sell tickets speak English, so it’ll be painless to buy a ticket. Departures are frequent (about every 5-10 minutes), and the journey time to the first stop within central Paris (Gare du Nord) will be roughly 30 minutes (some trains make a lot of stops, while others are non-stop). Here are two important points: The ticket that you buy for the RER will allow you to transfer to the metro that takes you to your final destination. Don’t discard that ticket until you’ve arrived at the metro station closest to your final destination, and actually exited the station! The cost for the RER will be roughly 8.5 euros each way.
Air France has buses that cost roughly 15 euros each way, run about every 20 minutes, and take 30-40 minutes to central Paris. Stops in Paris include Etoile (Arc de Triomphe), Gare Montparnasse and Gare de Lyon, which are major train stations. From those stops, you’d still need to get to your destination—- and this time, if you need to take the metro, you’ll have to buy a ticket.
There are also buses that go to Opera and Nation—- but for most visitors, the RER or Air France bus options should work well.
Get a good map, and study the metro system and get some idea about where you’ll be staying, which metro stop is close—- and if you’re not clar, repost!