Good evening. There have been a couple of incorrect replies in this thread- well meaning I'm sure, but incorrect, and following their advice could cost you some unnecessary $.
Homebuilt Parking- it costs you nothing to park your airplane at Oshkosh, as long as you park in one of the designated parking areas. You indicate your desire to do so by showing a "HBP" sign to each and every orange-vested volunteer you see from the time you clear the runway to the time you pull the mixture.
Homebuilt Camping- this is a subset of Homebuilt Parking, and you are allowed to pitch a tent and camp with your airplane in this area, which is awesome. The catch? It ain't free, and you will be charged the going rate for camping at Oshkosh, which I think is around $30-40/ night. It's the same cost if you camp in Scholler (which I do), North 40 camping, Homebuilt Camping, Vintage Camping, or anywhere else on the field which contains the word "camping."
"The Island" is the informal name given to a small subset area of Homebuilt Camping. If you park there, you will be charged for camping, whether you pitch a tent or not.
The maps which others have posted on this thread mostly incorrect as to the boundaries between camping and parking areas. But that is not something you need to worry about in advance. If you come in an RV and show a HBP sign, you will be directed to park with the other RVs. If you show up in a T-18, you'll be parking with the other T-18s. Long-EZs with the other Long-EZs. You get the idea.
The boundary between HBP and HBC is fluid year to year; we move it as needed to accommodate everyone as best we can. Generally speaking, HBP starts at the west end and fills east. HBP starts at the east end as works west, as somewhere around Monday afternoon they meet in the middle and we have a better idea of where to draw the final boundaries.
Another thing not directly related to parking- if you plan to come before noon on Monday, you could be arriving during some of the busiest times of the week, which is no where to be, especially if you are new to this. If you can swing it, get there early Monday, like before 8AM, when it is much less busy. A good option (depending on your travel distance) is to fly the day before to within an hour or so of OSH, overnight, and get up early enough to launch to arrive between 7-8AM. The recipe for success is to arrive at RIPON during a slower time, with full fuel tanks and an empty bladder, not the other way around.
Read the NOTAM. Bring good tie-downs.
Good luck.