Samsung Pay will formally launch in the United States late next month, but if you want to try it before that, there's now a beta you can register for. It's open to anyone who owns a Galaxy S6, S6 Edge, Note 5, or S6 Edge+ on all major carriers except for Verizon. If you've got AT&T, T-Mobile, Sprint, or even US Cellular, you're eligible to take part. Unfortunately, the nation's biggest mobile provider is only willing to say that it's "evaluating" Samsung Pay at this time, and Verizon could very well miss the September 28th launch.
You'll also need a debit or credit card from Bank of America (Visa / MasterCard) or US Bank (Visa only) for the beta; Samsung is obviously hoping to add more financial partners in the coming weeks. Apple already has a huge head start. Just because you meet the requirements doesn't guarantee you'll get into the Samsung Pay beta immediately; Samsung warns that it's "accepting a limited number of participants at this time." If you get in, you'll receive an email in whatever inbox is linked to your Samsung account.