A Paris Visite ticket covers the central 3 zones of the Paris transport system. (There's also a 6-zone version but, unless you're including travel to or from an airport, it's unlikely that you'd need it). It costs �8.80 for one day, �14.40 for 2 days or �19.60 for 3 days and comes with some discount vouchers for certain attractions. If you're travelling on Eurostar, you can buy Paris Visite tickets at St Pancras, Ebbsfleet or Ashford.
You may well find that you only need to travel with Zones 1 and 2. In which case get a Mobilis ticket (which is what the locals use). They give a day's unlimited travel for just �5.80. (If you require 3 zones, but don't need the discount vouchers you get with a Paris Visite, buy a 3-zone Mobilis for �7.70. There are also 4, 5 and 6-zone versions available).
When travelling around, always use the RER trains (in preference to the Metro) where possible. They're much quicker and tend to be less crowded at peak times.
As to what must be seen, the Eiffel Tower probably tops everyone's list. (Also view the Palais de Chaillot, directly across the river, while you're there). The Louvre is also popular but, with 18 miles of corridors, somewhat exhausting. The star museum is the Mus�e d'Orsay
http://www.musee-orsay.fr/en/home.html
Chris