It is completely up to you. But, if you want to play rated OTB chess in the US you will have to join US Chess (formally called USCF). Just let them know you are a returnung member and they will assign you the same ID. There is also the option for online rated play here and on ICC.
You could also play on FIDE Online Arena and get an official FIDE online rating. In general though, you join a national federation and then if you play in a FIDE rated event you could end up with a FIDE rating (if you meet all the requirements).
You don't need to join any federation if all you want to do is play online and don't care about "official" ratings.
[message deleted]