Naturally there are excursion buses which may take you all over the city. You board one settle down, stick in earphones with the guide talking non-stop in English, and spend a day sightseeing. This gives you a good idea what is really where so to speak, and a rough estimate of the sheer numbers of the historical places and monuments worthy of a visit.
When you have a little more time, there is a much cheaper and more enjoyable way of exploring this inimitable city. First buy a seasonal ticket which allows you to use all transport for a week or for a month. Then use it. I took bus 21 on Rue Gay Lussac, got out after two stops, went across the Luxembourg Garden, on to Rue Vaugirard which I now recognize, and then strolled along Rue Napoleon Bonaparte, looking at the beautiful buildings and snapping pictures. In a few minutes, with considerable surprise, I found myself in front of Saint Germain de Pres church! I walked all around and then inside. Afterwards I simply explored several little streets nearby, being careful to mentally mark a few landmarks. I found a pretty shop where they sell outrageously expensive, beautiful and extremely sweet macaroons, which are not macaroni but a kind of puff pastry made in all shapes and colors. It is certainly necessary to taste them at least once.
On another little street, I saw a lovely croissanteria with a wide selection of the famous French croissants. My mouth watered, and I thought of buying a couple. As it happens occasionally in any country though, the moment I finished studying the shop window, built my request in French, came in, the young woman behind the counter immediately turned her back on me and pretended to be very busy with something else. I left. Last time I checked, there are boulangeries, patisseries et cetera to be found on every corner. In many of them the staff speak English. And most of them are very friendly.
What happens next day? Why, you board the same bus, and travel through several stops until something new and inevitably beautiful catches your eye. Just alight and explore – and be sure to check that you know how to get back!