I went there for the first time last spring shortly after purchasing my Jeep. I had so much fun that I went back a few weeks later. I am looking forward to going back again this year, hopefully several times.
There is some basic information about the park on the DNR website along with a map of the park.
OHV Trail Detail: Minnesota DNR
You have to have your vehicle registered as an off-road vehicle. Real easy to do at the license bureau. If I recall correctly it cost about $45 and the registration is good for 3 years.
As far as the park is concerned, there is no admission fee (just the above registration requirement). Just check in when you arrive and always make sure to check out when done. That way they know that everyone is out of the park and not broke down somewhere. They will do a quick vehicle inspection if it is your first visit for the year, mainly checking that the sound level of your muffler is below a certain threshold. (The inspection took about 45 seconds). They will put a small sticker on your vehicle (if you want them to) that says that your vehicle was inspected and then on subsequent visits you don't have to go through the inspection.
There are trails for every experience level. Some are just like minimum maintenance roads and some are full-on rock crawls where you will need a modified vehicle to get through. Some of the trails go up on top of the large piles of tailings and there are some good views up there. The difficulty level of the trails is listed on the map and most of the trails are marked pretty well with signs.