You don't need to tell them a GPA, because obviously you don't have one. However, they'll calculate one for you once they receive your application, so you need to send them a complete highschool transcript, which has all your end of term/final exam grades from Year 10 to Year 13. There's a good example on the Fulbright website.
If you want to calculate it yourself, use this point system:
A = 4, B = 3, C = 2, D = 1, F = 0.
Add up the points for all the grades given, and then divide by the total number of grades. In technical terms, this is an unweighted GPA, where there's no extra points given for harder classes, because it's simply impossible to calculate a weighted one (where you get extra points for things like AP classes) with our system. 4.0 is obviously the highest you can get. I wouldn't worry about it, though, because the universities will be very familiar with the British qualification system.