Sign up with every legitimate company you possibly can, Do not EVER EVER EVER pay anyone money to let you shop for them no matter how exalted or special they tell you they are, no matter the BS they give you about they will offer you shops the rest of us never see. It's a bunch of crap to get your money in their pockets and to make her car payments. (Yes, I get stirred up about that subject but i digress......)
I have no certificate at all, nor have I ever paid anyone to let me shop and I've been doing this for 15 years now. A certificate doesn't make you better or worse, you have to learn by experience. Read this and other boards and learn all you can, sharpen your acting skills and make sure you can outrun goats and irate gas station managers and suchlike. I've been chased by both.
Don't be afraid to ask questions of other shoppers on the boards and if you post at one in particular, watch your back.
Her Serene Majesty, Cettie - Goat Queen of Zoltar, Sublime Empress of Her Caprine Domain