The Paris Métro is the quickest and easiest way to travel around the city, as well as the best value. The public transport network includes RER trains which serve both central stations and suburban stops, like Disneyland Paris or Charles de Gaulle airport.
Your Eurostar ticket doesn't include travel on the Métro or RER. You can buy your ticket when you arrive in Paris at a ticket desk or at one of the multilingual ticket machines near any Métro station. (Tip: you must keep your Métro or RER ticket with you until the end of your journey.)