Paris St Lazare train station to Kremlin-Bicentre metro line 7

You could walk underground a ways on a zig-zag path to Opera, or take the Metro No 3 line to Opera and change to the No 7, or take the No 14 line to Pyramides to get to the No 7. Your pick.

Do you want to go by mass transit? If so, take Metro Line 3 (direction Gallieni) to Opéra and change there for Line 7 (direction Villejuif-Louis Aragon) to Le Kremlin-Bicêtre stop. Otherwise, there are taxis available at Gare St. Lazare.

There is a metro stop at Gare St Lazare you just need to take that to a point where it corresponds with M7. or take a taxi.
This site is all you need to search if not a taking a taxi
http://www.ratp.fr/en/

Sam gave you the best options. If it is a nasty day take the underground walk to Opera following signs for metro line 7. If not then walk to Opera -- very pleasant walk -- or take the 3 from St. Lazare and change to 7 at Opera.