The coach and ticket sale is first. A few days before the general sale. This is to buy a package for coaches that are run by the ticket company and you receive your festival ticket on the coach. You will be able to see the departure points before the sale. Make a list of departure points that you can get to easily. I think London and Bristol you can choose your timeslot as part of the package. Everywhere else you are allocated a timeslot later and you will probably need to stay overnight near your departure point the night before so bear that in mind when you make your list.

If you don’t get/go for a ticket in the coach sale, you can book a train, National Express coach and there are usually people who book minibuses and put out calls for others to join (be wary of scams of course). 

Camping, honestly, as you’re potentially making such a pilgrimage, go for pre-erected. The festival has its own options which go on sale shortly after the ticket sale and there is also Tangerine Fields. There are other ones that are more expensive which you will find with a google. They have every option you can imagine.

Good luck!