Most states have a “Good Samaritan” statute which protects untrained responders who provide help in an emergency. While they all differ slightly in the detail, basically what is required is that if an individual expects no payment, and only does what a normal person would do under the circumstances, they are protected.
However, certain states have rules about the use of AEDs, which in some cases require designated users to have CPR/AED training. This training is typically part of a standard First Aid/CPR/AED class, which is provided by the Red Cross, American Heart Association, National Safety Council, or any number of local sources.