The links above will get you the official info. Here's the basics: You'll need to purchase and display a WMNF parking pass on your car to park in any of the Forest Service controlled lots. Not all lots are FS controlled, some like Lafayette Place Campground are state owned and aren't part of the program. I don't have a full list.
You can purchase a full year pass either directly from the FS or from many vendors in the area. Some places near Boston, like REI, sell them as well. The pass is good for one full calendar year from purchase, and costs $20 for one or $25 for two. You can also buy a week long pass for $5. Some trailheads have self-service kiosks for single or multi-day tickets for $3/day.
They are now ticketing cars at the trailheads, so it pays to get a sticker. Most of the money goes back to the national forest, so it is being used for the trails and rangers. It's not the best system, but it is a good one.