I visited Chartres from Paris' Montparnasse station, accessible using the surface rail (RER). It only takes 50mins or so either way, and the train says Chartres on it clearly. No problems there. The station staff should speak sufficient English to point you to the correct platform.

At the station, use the underground pathway to get to the main station, don't cross the tracks.

The cathedral itself is a 10 minute walk from the station. It dwarfs anything else in the city, and is so awesome you will probably not notice anything else. You can see it looming in its magnificence from the station, then walk there following your view. You can't go the wrong way, don't worry. On the walk there, there is a nice statue and square.

There is no entrance fee. It's the local cathedral, you see. You can go up to the deep dark bell tower, but you better be early because it closes at 3pm. This is the historic Notre Dame de Chartres, Gothic, magnificent and even more atmospheric than its sister in Paris. I have also been to Cologne, Milan and St. Peter's in Rome Cathedrals, this is as 'prayerful' and 'cool' inside, and much more peaceful because there are usually not as many tourists. There are the usual gargoyles sticking their heads out of all corners outside.

There is a plaque at the rear in French, justifying its inclusion as a World Heritage Site. Read it yourself, I will not spoil it for you.

The view of the city from the rear is not spectacular, but its worth walking back there to see the museum and the view of the cathedral from the back.