Dont, youre only 13, you have to be 18 in order to use money pack.. Now that you mention it, I actually brought a money pack before I was 18, and they charged me 5 bucks a month (not sure if that cost has gone up).. long story short I forgot my password and I have no idea how much money I owe them by now, luckily it hasnt affected my credit score, I cant even remember what email I used to sign up with, lol.
Dont do it. If you want to buy online you can always buy a gamestop or amazon gift card, which dont require any monthly fee or personal information like an SSI.
EDIT: Ok, yes, they charge you every month for the card, but only if you activate it and to do that they need your social security number or they cant charge you for the payments. And YES they will know everything about you if you give them your SSI including your age, but they will not check your age, because they really dont care. You best option is to throw the money pack in the trash.. where it belongs.
Like I said before, if you want to buy online then just use an amazon gift card, anything youre looking to buy is on Amazon.com TRUST ME. Throw the money pack away or something, its not worth the hassle.