Margaret,
if you dont want to do it yourself then JY can arrange it, but it around 3 times the cost of just logging onto the US offical website and spending 5 minutes completing an online form, which only costs about £9:00. I prefer to do this because I'm in control of it and I can print a copy of it and keep it with your passport(although you dont need to, its peace of mind).
Up to you which you do, but if you do it yourself heres the link:
https://esta.cbp.dhs.gov/esta/