If you are going to use the Metro, research where you need to go first to familiarise yourself. There's nothing worse than arriving somewhere and not knowing where you need to go. The Metro lines can be seen on Google maps and while it might not appear very direct, they are quick and the trains are frequent. The train stock is old and it shows, but it works. You have to open the doors yourself, both to enter and exit. The lines operate on a colour and number system. Tickets have to be bought before you get onto the platform. The machines have an English option, as well as German and Spanish, but English didn't work. Fortunately, we are reasonably competent, but still had to go through it two or three times to get the ticket. There are staff there to help, or you can go to a ticket office if it is open. You can buy a single, one journey ticket for two euros. Or, if planning multiple journeys, for another two euros, have a reusable plastic ticket with the amount of journeys required. This can be topped up.
There are stairs and escalators at the entrance/exit. We didn't notice lifts to help disabled, but might have missed them. We arrived at Paris-Montparnasse and the signs were easy to follow to our Metro line.
Toilets at Montparnasse cost one euro to use and were clean, but busy. You can use coins or card.