We are staying at 29 Rue Cler, Paris. We plan on shopping the flea mart at Vanves on Marc Sangnier Blvd. I understand that line 13 on the metro will get us there, but I am not sure what station we should enter to take us, and how do we transfer from line 8 to line 13?
Change from 8 to 13 at Invalides.
You should look at the Rick Steves Paris book as well as The Paris by Train website for Metro basics. Looking at a Google map, I see your accommodation is about equally close to the Ecole Militaire and La Tour-Maubourg stops on line 8. Then, you should look at a metro map while reading the following: You enter at either one of these stations, and take the train labeled "Direction Creteil Prefecture." You take it one stop (from La Tour-Maubourg) or two stops (from Ecole Militaire) to the Invalides stop. At this stop you follow signs labeled "Correspondences" to line 13, and to "Direction Chatillon-Montrouge." You take this train to your desired stop (I'm not sure where that market is) and exit (follow "Sortie" signs). If you don't mind a slightly longer walk, you can walk to the Varenne stop on line 13, so no changing is needed. Your cheat sheet: "Direction" indicates which way the train is going, by telling you the last stop (not easy at first, but you'll get used to it). "Correspondence" means transfer - the way to another line (or lines).
"Sortie" means exit.
Harold has given you excellent directions. One minor correction: The French word for connection is spelled correspondance. For a zoomable map of the Metro and RER system, go here: http://tinyurl.com/6xmvms4. It looks like the closest Metro station to the flea market is Porte de Vanves.