From Kampot, take the road towards route 4 about 8 kilometers.  On the right side, you will see a large plaque welcoming you to Bokor National Park.  Just up the road is the Bokor ranger station.  There is no fee to go up the mountain.

You can take a motorcycle or car up to the top.  If you don't have your own bike, big dirt bikes can be rented in Kampot for less than $12/day and 100cc motos for $4 - $5.  Gas is $1 a liter.  The drive up is now smooth and quite nice.  You may want to bring some warm clothes (and a raincoat) as the temperatures, especially at night are much cooler, and there is often fog or rain at the top.

You can also get a ride in a pickup truck (either inside or in the back) for a day trip, including lunch for $18 to $25. Most tours leave from downtown Kampot.

Walking is sometimes an option, but plan for a few day trip, with lots of gear.  You'll need to check at the ranger station at the bottom of the mountain, to see if they'll let you walk.  If you get far off the beaten path, beware of wild animals (and illegal poachers).  You can hire the rangers to give you a tour, and the price is whatever they can charge.  $10+ per day.   There are walking trails on top from a couple hour walk to a full day.

There are some sights to see on the way up, and at the top, such as the waterfalls, the Bokor Hydroponics Farm, and the old King's "staff" accommodations, the old Church, and the "Old Casino"", so you might want to take a few rest breaks along the way.  (and stop at the "Lookout Points" as you get higher and higher". You should also bring something to eat and drink, which is really helpful if your vehicle breaks down.

Most people stay in hotels in Kampot, Cambodia, and take the day tour up the mountain.  That's changing a bit now, with the new Resort at the top.  Though rooms start at $60 or so.  You can also take the trip from Kep, Cambodia, but it's about 60 minutes more travel time, round trip.