Sometimes there are lanes marked for bikes, but I just go to any lane marked for card payment, manned or not.
Sometimes the tolls don't always recognise you as a bike, and sometimes the price is the same as a car anyway.
I just insert my ticket and push the help/telephone button. A French voice will then say something and I just say, "bonjour, J'ai un Moto?!?!"
Which translates to, hello, I have a motorbike. ( "j'ai" rhymes with hay.;-))
In the unlikely event you get a confused sounding voice on the other end you may need to say this a couple of times more slowly and/or more loudly.. Last resort just say it in English. ;-)
At this point one of three things will occur: 1. They will change the price and you pay as normal. 2. You hear the French voice again but this time sounding a bit more negative. In this instance assume there is no discount for a motorbike and so pay as normal. Or 3. You get lucky and they just open the gate and it's free
I then say, "merci!!" Either way and ride off after paying.
Sometimes if there is a big bike event on, at Le Mans for example, there is a bike lane to the right which you can go through for free.
Have fun! keep polite, smiling and friendly with the French and 99 times out of 100, they'll help you!