Its good to see the frame has some lugs on it so you can fit a rack, get the best you can afford, that will make carring your equipment easier.
Its also nice to read that guys your age and your mates age are planning to cycle tour.
Firstly I would say have some pratice rides maybe 30 miles over a weekend, you can camp in either yours or your mates garden.
Once you have sorted out any problems, with bike and equipment, take a ride out may be 30 odd miles to a campsite, however in this day and age most sites dont want to know under 16 Year olds unless they have a adult with them. but there is nothing wrong in finding a quite place to wild camp, camp early and leave early and normally you will not have any problems.
browes through past postings on here to see the vast amount of useful information that is on this forum.
good luck and enjoy.
Bob G.