Chartres is an easy hour's train ride from Paris. A train leaves from Gare Montparnasse every hour. It costs about $20 for a one-way ticket. Even if you buy a France Rail Pass, good for any three days of travel in the country in a month, you will come out ahead if you buy a separate point-to-point ticket to get to Chartres. Save that Rail Pass for longer treks.
While Chartres is an easy day trip from Paris, I suggest buying a one-way ticket and spending the night in Chartres. Not only will you find a fine hotel for half the price of one in Paris, but you'll have the benefit of seeing the Cathedral's stained glass windows in all sorts of light, and you'll have time to really soak in the ambiance of Chartres. Check out the flea market or the city center boutiques, or, rest your feet, and take in a movie. Who needs English subtitles?!