A visa is required to enter the United States for many nationalities. Please check with your local U.S. embassy/consulate whether this applies to you. In case a visa is required, please apply as soon as possible, as obtaining the visa may take several weeks.
The United States Department of State provides a list of countries that are participants in the Visa Waiver Program here. If you qualify, you will not need a letter.
If you need an invitation letter to PPoPP 2017, you should register for the conference first and then contact email@example.com. For more details on procedure, please visit this link: http://www.acm.org/sigs/volunteer_resources/conference_manual/visas .