Welcome! If you read this board you'll find a wealth of information here. Legitimate companies to sign up with are listed here and remember, you do not EVER EVER EVER pay anyone to let you shop. They are supposed to pay YOU, not the other way around. Also, much is said about getting "certified" but you get the same shops offered to you regardless of being certified or not. The only difference is that to call yourself certified you give money to the MSPA and it goes right into their pockets. I'm not certified and have been shopping close to 18 years now.
Good luck!
Her Serene Majesty, Cettie - Goat Queen of Zoltar, Sublime Empress of Her Caprine Domain