You are strongly advised to take out travel insurance before you visit the States. There is no NHS in the country and medical care is prohibitively expensive.
Most travel agents offer reasonable insurance, but do check that it covers all eventualities.
Check the level of insurance covered by your bank or credit card and make sure that it's adequate. If you suffer loss or theft, make sure that you contact the local police so that they can complete an official report for presentation to your insurance company. When in the States, remember the emergency services telephone number - 911. If you have, or witness, an accident or other medical emergency, a call to this number will normally result in prompt and efficient service, though you will be expected to pay later for any treatment. If you need a doctor, look up 'Physicians and Surgeons' or 'Clinics' in the Yellow Pages.