Nevada was the first place that had legal betting in the state, while New Jersey, Pennsylvania, and Delaware all allow sports betting in casinos. Arkansas, Mississippi, New Mexico, Washington D.C. , and West Virginia have physical sites currently, or in the process of opening.
Online is a different story. As of February 2019, there are five states in America where it is legal, at least in part: Mississippi, New Jersey, Nevada, Pennsylvania, and West Virginia. The law in Mississippi is a little strange, as it has to be done either in a casino or on a riverboat, and only one tribal casino has launched it as of February 2019. Pennsylvania will all out its online betting sites sometime in the spring of 2019.
The Garden State, along with its 10 physical sites, currently has 13 online sites available to use, with that number expected to rise. Draft Kings Sportsbook was the first site to take bets on August 6, 2018, followed closely by Sugar House Online Sportsbook Casino (which was the first gaming operator to have an online site.) The next two to open were FanDuel and 888 Sports. BetStars is one that has very good reviews, and is very fan friendly, as is the FanDuel site.