It doesn't really matter which ferry crossing you take, just pick whichever is closest or cheapest.
You'll need to start booking accomodation up very soon if not now, regardless of whether you want to camp actually at LeMans itself or at one of the various campsites located around the area, or if you want to be really boring and stay in a hotel.
General entry tickets for the practice/qualifying/race can be bought on the door, but if you want a grandstand seat then you need to move pretty quickly as they are selling out fast this year. Go via Just Tickets if you want these, good people to deal with. They'll also sell you the car parking tickets, ideally you want white if not red if you're not staying on site.
Download this guide for last year, contains everything you could ever need to know. This year's one will be out in the next few months but you'll want to be sorted by then more or less, and the info in last years is 99% the same as what will be in this years.
Above all else, just go! The weekend is whatever you make of it, do your own thing and you'll love every second of it.