(Help) Buying Xbox 360 Live (Help)

I didn't know what section to put it into for console gaming help.
I've put a new credit card onto my xbox 360 account and changed all the details etc but I don't know where to buy the xbox 360 live time from. Can someone tell me step by step directions on where to purchase it on the xbox dash or online?

 

I'm on my phone right now, but I'll be home soon. If you havent already figured this out, shoot me a PM and I'll walk you through it when I get back.

 

I think the only way to add time without buying the physical card from the store is to run out of gold time, and while having a silver membership, there should be some kind of option for "upgrading" or "upgrading to gold".