You can buy metro tickets at a machine next to the turn styles in most stations. Be sure to hold onto your ticket until you exit the metro station at which you arrive, as you may be asked to present it at any time, even after leaving the train and walking to the exit. If you don't have it, you receive a fine.
Every metro station has steps down to and up from. Some have escalators, at least one, up by Montmartre that is five stories underground, has an elevator. I mentioned Uber or taxis because you said you had issues with walking.
If you take the metro, you will be mostly underground, seeing nothing. There is a public bus, #42. The 42 bus route begins at the Gare du Nord train station and cuts diagonally across Paris going through Place de la Opéra, past the grand Garnier Opéra, around Place de la Madeleine and Église Madeleine, through Place de la Concorde, up the lower Champs-Élysées, across the Seine on the Pont de l’Alma bridge, along the Left Bank quais of the Seine, through the Champs de Mars and around the Eiffel Tower. (Copied from the web site mentioned below.)
Bus #69 is another great route, going west to east across the city. Try this site and click on the links. It's a web site for apartment rentals, but it has useful information.
https://www.parisperfect.com/blog/wp...original-8.jpg