The RATP is the authority that runs ALL public transport in Paris. It has a really good web site that I've just looked up for your very short and simple trip -
http://www.ratp.com/. Click on the Union flag for the English version and navigate through the 'Finding your way' section. Put in your departure and arrival points (click on the little dot for station) and it'll come up. There's an RER (Paris suburban ) line that runs from Magenta, right next to Gare du Nord, and will take you in 4 minutes to Saint Lazare.
Superb public transport system - properly managed and not privatised.
Bon voyage,
Thorfinn Johnston
Glasgow