These days when you make the appointment with the US embassy they take the fee off you there and then.
my advice is:
1.Get sorted with the school you want to go to and get the I-20 usually around $100-$200.
2. Once you have the I-20 make your Visa appointment and pay your fee $100. (The phone call costs about £8 because they use a premium rate number)
3. Pay your SEVIS fee $100
4. Open an account with the TSA but do nothing other than open the account.
5. Sort out where you will staying whilst training (it is needed for the visa form)
6. Visit embassy and await visa (£13 delivery Charge)
7. Once visa received go to TSA site and submit training request. This process will cost around $100.
8. Once the request is approved, finger prints are needed. These can be done in the UK at Flight Safety at Farnborough for $130 or wait until you arrive in the States.
Basically dont arrange for your visa interview until you have all your info together.
Currently the US embassy in London is busy processing all the camp america applicants so getting an appointment may mean that you get a cancellation at short notice or have to wait a bit. Having all your paperwork sorted means you can get the cancellation. ( I got a cancellation and only had to wait a month).
The other point to note is only take yourself and you paperwork to the embassy. No mobiles no laptops no mp3 players nothing electrical at all and no big bags. You will be refused entry and you might have to pay the appointment fee again $100.