Join any casino, which is listed here at OC. I think, there is no any problem for USA members to play games.
Just do a google search with the term: Online gambling sites that take players from america. This should bring back a few results. As for the credit cards, look at each site that does take players from the U.S.A. and see what payment methods they take. Often, prepaid cards that you buy will only be for purchases from companies inside the United States, until you get your permanent card. The one you receive in the mail should allow for international purchases, and as most gambling sites are outside the U.S., this might work for you. I used the highest prepaid Visa from Walmart that charges no fee to reload. Read each card carefully as to what details and restrictions involving both national and international purchases, as well as fees for reloading, that it may have.
I do very much agree with @Emmit as they should read all the available places to play online. And yes the Casinos that OC have refered to us is a good starting place for you.
I also agree that with the pre-paid cards that most US Citizens use they work only after you get the perminent one from the bank issueing it. As the temporary ones you get with the purchase is not for use outside of US, only the perminent one is.
And please do make sure you read the rules and regulations on the Casino site before attempting your deposit. Also if you have questions get in contact with the Casino site chat or phone them they are always happy to accept your money.
And last but not least, I want to introduce everyone to one of dear friend @Emmit. As he is a experienced online writer and is very happy to come help out here on OC. He comes wiht plenty of knowledge on writing and playing poker online. His knowledge will help you in ways that I cant even do.
And I would like to introduce @Emmit to @admin and hope you would take time to get to know him like you have done with me.
In short words, yes online gambling is legal in USA. @Emmit is correct for what he said for your problem.