First, the Chunnel is an out-of-date term for the Channel Tunnel, which you can't 'take' anywhere because it doesn't move.
What you take is a 'Eurostar' train ( http://www.eurostar.com/ ) which runs from Paris Gare du Nord via the Channel Tunnel to London St Pancras station and vice versa. Click here for the full timetable, but, simplified, there is one train per hour in each direction, taking between 2¼ and 2½ hours, usually non-stop. First train leaves Paris 07:13, arrive London 08:32 (1 hour time difference to your advantage).
You could book the train directly, in which case book 2-3 months in advance to get the cheapest prices. But, there are also companies which combine a train trip with a guided bus tour of London, then some free time before you have to get back to St Pancras. For first time visitors this saves lost time getting orientated, but cost more than booking just the train tickets yourself.
Google: "day trip london paris eurostar", or there are often leaflets advertising these tours in hotels in Paris.