It depends on what you enjoy eating. Do you like sandwiches and crepes? You can get sandwiches at boulangeries and crepes at either sit down creperies in Montparnasse or creperie stands all over Paris. Cafes are on every corner, and there are small, family run restaurants as well as lots of Michelin star restaurants. I would suggest starting with Chowhound for good recommendations. We enjoy the Flo Brasseries restaurants, Brasserie Balzar & La Coupole, and you can book them online:
Hint one: Google "fooding". Fooding is a movement in France that promotes classic french food in a relaxed bistro setting at affordable prices. There is a good Paris restaurant guide under fooding.
Hint two: Bring a french phrase book, know what you are ordering. The results will be better. Generally, the better the restaurant, the less english will be on menu and spoken by your waiters.
Oh and do not eat at any place that you could eat at back home.
For chocolate: When in Paris, it is ESSENTIAL that you drink the fabulous hot chocolate in ANGELINA'S across from the Jardin des Tuileries(1st arrondisement). It is indescribable what it tastes like. I have had hot chocolate in many places and have tasted chocolate everywhere like Brussels, or even Hershey but it was amazing! It wasn't a liquid really but it was not too thick or overwhelming or too rich that made it so you could not drink it. I only like milk chocolate so it had that delicious taste. I shared a mini pitcher with two other people at my table. Nearby, if you go out of Angelina's, turn left, go down the street and along that street across from a patisserie with automatic door is a chocolate shop called Michel Clunzy. I went there to try what those at the time foreign macaroons were, I figured out that the color of it corresponded to their flavor, and ordered a chocolate one because I wasn't in the mood to try a strawberry, lemon, vanilla, hazlenut? I tried it in the store while glancing around at the elegant chocolate fountain and the array of chocolates and it was amazing. By far my favorite dessert ever and I knew it at that moment! It tasted a little like a brownie but so much better. The macaroon was perfect. In taste and shape. Later on on my vacation I went to Brussels, and got a package of macaroons there, and nope- macaroons are purely Parisian. Anywhere else, was nowhere near to the real deal.