There has been plenty already on this and if you hold British, or other eligible, citizenship, you don't need a visa as you may enter under the tourist visa waiver program(me) as a PPL course is recreational, i.e. non-vocational, training. Just fill in the green form which will be handed out during the flight from UK. However, the visa waiver does not apply if you intend to take vocational, includes commercial flight, training or, of course, work in the US, in which case you must have the appropriate visa, before you enter.