I reserved in Grand Hotel Leveque and I want to take the bus 42 to have a tour in Paris. from which station should I start if I want to see all the city as much as possible and how can I reach this station?
There's a stop on Avenue Joseph Bouvard just west of Avenue de la Bourdonnais. which is 10 minutes walk from your hotel.
The name of the stop is "Rapp-la Bourdonnais".
But, IMO, it's not an efficient way to see the sights; plain city buses are not made to make it easy to see the sights (smaller windows, lower, full of regular people).
jcaron, in the comments suggested using the hop-on hop-off tourist bus; which would be easier and more practical.
Anecdotal, as someone who've been to Paris lot of time, it is sometimes fun just relax and take a regular bus and just do people watch.