If you want to switch between the "real money" and "play money" modes, then this can easily be done in the lobby of the poker room. Download PokerStars and log in to poker client. In the main lobby, on the upper right corner, you can see tabs "Real Money" and "Play Money".

Real money online poker is legal and available in New Jersey, Pennsylvania, Nevada, Michigan and Delaware. It's also legal in Connecticut and West Virginia, but yet to go live there. The top NJ online casinos and Pennsylvania sites are the leaders in online poker.