Lille has a good transport network with metro stations dotted around the city.
If you don't have much time but would like to see most of the sights and learn some of the history of the city, take one of the tourist office tours. They leave hourly and are in English, French, German or Dutch. The tours take 50 minutes and you listen on a microphone with video as well. It costs 8 Euros (no stopping to get off, only slowing down for photos) and is a great way to see as much as possible.