If you are in the USA it seems that today you can only play at PokerStars from New Jersey. But I think only against other people from Jersey.
From a Google search:
There are currently 3 states that offer legal online poker in the United States
- New Jersey, Nevada, and Delaware. Pennsylvania is scheduled to join this group in 2019.
WSOP/888 US is currently the only poker site that shares online poker players between all three legal online poker states (NJ, Nevada & Delaware). It’s currently the #1 US online poker site by traffic.
Poker Stars – New Jersey only
Party Poker – New Jersey only
Pala Poker – New Jersey only
There are other sites that accept US citizens:
BetOnline Poker
Ignition Poker
Intertops Poker
Americas Cardroom
Bovada Poker
BlackChip Poker
SportsBetting Poker
True Poker
The later list is "gray area - sites". Read up on them before you decide to play.
Link to further info:
https://upswingpoker.com/us-online-poker-sites-update/