Hey there! I’d inform you with pleasure! ;)
First of all, be sure you’re not claustrophobic. That’s the very first step since you’ll be around 20m under the ground, in looong corridors! Now for the adress and other practical informations: The metro station is Denfert-Rochereau on the RER B, and the metros 4 and 6. There are several exits but they’re all graviting around the catacombs entrance so it doesn’t really matter which one you’ll choose ;)
There’s one just facing the monument though.
Now about the time to go there. Come REALLY early. When we’ve been there with my friends, we showed up at 9am and only entered 2hours later. The queue is always super full especially during the weekends and national holidays (the catacombs open at 10am and close at 4pm so you really need to organize your visit).
The estimated time of the visit is 45min… but in the end we’ve stayed down there for 2 plain hours :0)
The visit costs between nothing (for teens) and 10€ if my memory’s right.
Don’t go there well dressed. I’ve seen a Gothic-Romantic fellow, thinking it would be super romantic to show up in the Catacombs with frills and lovely shoes… she’s certainly still regretting it.
The stairs to get down may give you a slight vertigo since they’re long, spinning stairs… no there ain’t elevators x) (and that’s the same to rise back at the end of the visit.)
Once you’re down, and especially in the ossuary, be respectuous and turn your camera’s flash off. Don’t touch the bones (it might sounds stupid to precise this but I’ve seen morons doing it, so…), remember you’re not into a theme park but into an open cemetary, with real, visible corpses (that’s why I wouldn’t recommend this place for young kids)
I think that’s pretty much it! I hope those informations have been and will be useful! :) have a sweet trip in Paris and don’t hesitate to ask for more nice places to visit in the capital! ;)