Paris is divided into 20 districts called arrondisements. Go to this site and get a feel for which districts suit you. http://www.parisnet.com/parismap.html Is it important that you be near certain tourist attractions? Do you have business that will require you to be in a particular part of town?
Once you can narrow it down to one or two arrondisements, I would suggest tripadvisor to get some ideas for hotels. You can select a particular arrondisement and read reviews from people who have stayed there. Also ask yourself if you have needs that may limit your choices. Many hotels have no elevator. Do you care? Must the hotel have a restaurant? Some people insist on one, even though it seems there is at least one good restaurant every 10 meters or so.