(not sure where else this would go)
So i have a credit card that's tied to my mom's account and I just have one question. Will she be know what i buy from the bill?

If it is tied to your mom's account then it is called a trustee's account. She can see everything that you buy on the credit card, especially if it is tied to her account (trustee account).
~Vincent

It depends on the bank/CC provider. If its tied to your mums account, I would imagine she could, but that is just a guess.

I'd guess that it would show up on the bill.

I know in America it is called a Trustee account, where it is technically the "trustee's" until you are old enough to have it. If you are under 18, then your parents (or guardians) MOST LIKELY, are the trustees for your bank account. They can also freeze the account, or close it if they wish.
~Vincent

For my own debit card, I see things like "Valve Software", "McDonald's", "Paypal", "www.Skype.com", &c. Even tiny little places, like the local pizzerias, are identified by name. So to put it simply, anywhere you buy from is listed. And depending on where you buy from, she may be able to tell what you bought. Valve Software would be something or some things on Steam. McDonald's would obviously be lunch. Paypal would usually be eBay (though some subscriptions, like Imageshack, are also there) or Paypal itself. Then others like Skype would be credit for phone calls.

I guess you're trying to get RS membership without her noticing it, this appears when you get members:

WP-INFO.RUNESCAPE.COM WILLIAM MORRI

When I bought something with paypal, this appeared:

PAYPAL ANCHORBREAK [insert some numbers here]