Final answer:
The missing word is 'need,' indicating that British people typically do not need to get a visa for short visits to the United States under the Visa Waiver Program.
Step-by-step explanation:
The missing word for the sentence 'Before you travel to the US, you must find out what documents you need. British people do not <> to get a visa, but there are different rules for different nationalities.' is need. This is because British tourists traveling to the United States for less than 90 days usually do not need to get a visa thanks to the Visa Waiver Program. It is important for travelers to understand the immigration laws and visa requirements of the country they plan to visit. Always be mindful that circumstances can change, and requirements may differ based on various factors, so it's essential to check the most current regulations before making travel plans.